Binary Search Tree implementation with topological validation
Coding style
Python has some conventions about coding style, for example snake_case for variables and functions etc. You can find these in pep-8
Node.__repr__
for troubleshooting, this can be handy:
def __repr__(self):
    return f&quot;Node({self.value})&quot;

with optionally the values of the children elements too
BST.update
adding a simple method to add multiple nodes can make initialization a lot simpler:
def update(self, values):
    for value in values:
        self.insert(value)

It also allows you to do this immediately in the __init__
def __init__(self, values=None):
    self.root = None
    if values is not None:
        self.update(values)

and use something like this in your tests:
bst = BST(range(5))

Node
All of the methods you prepend with an _ make more sense as methods on the Node
_xxorder
for example _inorder:
def inorder(self):
    if self.left is not None:
        yield from self.left.inorder()
    yield self
    if self.right is not None:
        yield from self.right.inorder()

and then BST.inorder:
def inorder(self):
    return self.root.inorder()

You can easily foresee a reverse iteration too (for example to find the maximum of the tree:
def inorder_reverse(self):
    if self.right is not None:
        yield from self.right.inorder_reverse()
    yield self
    if self.left is not None:
        yield from self.left.inorder_reverse()

same goes for the _doFind. Node.find:
def find(self, key):
    if self.value == key:
        return self
    next = self.right if self.value &lt; key else self.left
    if next is None:
        return None  # or raise IndexError
    return next.find(key)

and BST.find:
def find(self, key):
    return self.root.find(key)

magic methods
isSameTree compares 2 trees. Why not name it __eq__.
Your implementation doesn't really use seld, so it might make more sense to transfer it to Node to compare subtrees
Node.__eq__:
def __eq__(self, other):
    if other is None:
        return False
    return (
        self.value == other.value
        and self.left == other.left
        and self.right == other.right
    )

BST.__eq__:
def __eq__(self, other):
    return self.root == other.root

You can easily implement the Iterator protocol on BST:
__iter__ = inorder

and reversed:
__reversed__ = inorder_reverse

Sum
You don't need the Sum class, you can just pass on a value. Also this method seems more appropriate under the Node class:
def sumRToL(self, partial_sum=0):
    if self.right is not None:
        partial_sum = self.right.sumRToL(partial_sum)
    self.value += partial_sum
    if self.left is not None:
        self.left.sumRTol(self.value)
    return self.value

Using this on mutable values might have strange effects.
on BST:
def sumelementsfromRtoLinplace(self):
    if self.root is not None:
        self.root.sumRToL()

validate
checking whether your tree is valid can become very easy via the iterator we just implemented. Using pairwise from the itertool recipes:
def validate(self):
    return all(a &gt; b for a, b in pairwise(self)) # or self.inorder() for extra clarity

testing
These unit tests can be better done in another file, importing this file, and using one of the unit test frameworks. I'm quite happy with py.test.
import pytest

from binary_tree import BST


def test_order():
    bst = BST(range(10))
    assert [item.value for item in bst.inorder()] == list(range(10))
    assert [item.value for item in bst] == list(range(10))


def test_reverse():
    bst = BST(range(10))

    items = list(reversed(range(10)))
    assert [item.value for item in bst.inorder_reverse()] == items
    assert [item.value for item in reversed(bst)] == items


def test_equal():
    bst1 = BST(range(5))
    bst2 = BST(range(5))
    bst3 = BST(range(6))
    bst4 = BST(range(-3, 6))

    assert bst1 == bst2
    assert bst1 != bst3
    assert bst3 != bst1
    assert bst1 != bst4
...


total code
from general_tools.itertools_recipes import pairwise


class Node:
    def __init__(self, value):
        self.left: Node = None
        self.right: Node = None
        self.value = value

    def inorder(self):
        if self.left is not None:
            yield from self.left.inorder()
        yield self
        if self.right is not None:
            yield from self.right.inorder()

    def inorder_reverse(self):
        if self.right is not None:
            yield from self.right.inorder_reverse()
        yield self
        if self.left is not None:
            yield from self.left.inorder_reverse()

    def preorder(self):
        yield self
        if self.left is not None:
            yield from self.left.inorder()
        if self.right is not None:
            yield from self.right.inorder()

    def postorder(self):
        if self.left is not None:
            yield from self.left.inorder()
        if self.right is not None:
            yield from self.right.inorder()
        yield self

    def find(self, key):
        if self.value == key:
            return self
        next = self.right if self.value &lt; key else self.left
        if next is None:
            return None  # or raise IndexError
        return next.find(key)

    def __eq__(self, other):
        if other is None:
            return False
        return (
            self.value == other.value
            and self.left == other.left
            and self.right == other.right
        )

    def sumRToL(self, partial_sum=0):
        if self.right is not None:
            partial_sum = self.right.sumRToL(partial_sum)
        self.value += partial_sum
        if self.left is not None:
            self.left.sumRTol(self.value)

    def __str__(self):
        return f&quot;{self.value} &quot;

    def __repr__(self):
        return f&quot;Node({self.value})&quot;


class BST:
    def __init__(self, values=None):
        self.root: Node = None
        if values is not None:
            self.update(values)

    def insert(self, key):
        if self.root is None:
            self.root = Node(key)
            return
        curr = self.root
        parent = None
        while curr and curr.value != key:
            parent, curr = curr, curr.right if curr.value &lt; key else curr.left
        if parent is not None:
            if parent.value &lt; key:
                parent.right = Node(key)
            else:
                parent.left = Node(key)

    def update(self, values):
        for value in values:
            self.insert(value)

    def delete(self, key):
        pass

    def find(self, key):
        return self.root.find(key)

    def inorder(self):
        return self.root.inorder()

    def inorder_reverse(self):
        return self.root.inorder_reverse()

    def preorder(self):
        return self.root.preorder()

    def postorder(self):
        return self.root.postorder()

    def sumelementsfromRtoLinplace(self):
        if self.root is not None:
            self.root.sumRToL()

    def validatebst(self):
        return all(a &gt; b for a, b in pairwise(self))

    __iter__ = inorder
    __reversed__ = inorder_reverse

    def __eq__(self, other):
        return self.root == other.root