Simple encoder by a beginner
I'd highly suggest learning comprehensions. The simplest is a list comprehension. This allows you to change say:


list = []
for x in string:
    list.append(dic[x])



To:

list = [dic[x] for x in string]


However, you can get better memory usage by using a generator comprehension, this works in roughly the same way as a list, however you can't index it. Which for this code is fine. To do this replace the square brackets with curved ones:

gen = (dic[x] for x in string)


You can also use a dictionary comprehension to simplify your creation of dic2:

dic2 = {v: k for k, v in dic.items()}




Purge all memory of eval, it leads to poor code and has severe security vulnerabilities. Instead cast the users input to an int. This can simplify decoding too. However you have to make sure the user does actually enter a number.

I don't like list = [] being outside the if statement. There's not much of a need for this. However it's fixed when using comprehensions.

I also think you should use while True rather than while program:. As you never change program.



Merging this all together gets you:

dic = {"A": 0, "B": 1, "C": 2, "D": 3, "E": 4, "F": 5, "G": 6, "H": 7, "I": 8, "J": 9, "K": 10, "L": 11, "M": 12, "N": 13, "O": 14, "P": 15, "Q": 16, "R": 17, "S": 18, "T": 19, "U": 20, "V": 21, "W": 22, "X": 23, "Y": 24, "Z": 25, " ": 26}
dic2 = {v: k for k, v in dic.items()}

while True:
    task = input("Would you like to encode or decode?: ").lower()
    if task == "encode":
        string = input("What would you like to encode?: ").upper()
        while True:
            try:
                key = int(input("What key would you like to use?: "))
                break
            except ValueError:
                pass
        list = (dic[x] for x in string)
        encodeNums = ((x + key) % 27 for x in list)
        encodeStr = (dic2[x] for x in encodeNums)
        print("".join(encodeStr))


Going forward, I'd change your code to encode and decode with the same code. This is as the Caesar cipher is a symmetric cypher, so simply using key = -key allows you to decode the encoding.

Allowing:

dic = {"A": 0, "B": 1, "C": 2, "D": 3, "E": 4, "F": 5, "G": 6, "H": 7, "I": 8, "J": 9, "K": 10, "L": 11, "M": 12, "N": 13, "O": 14, "P": 15, "Q": 16, "R": 17, "S": 18, "T": 19, "U": 20, "V": 21, "W": 22, "X": 23, "Y": 24, "Z": 25, " ": 26}
dic2 = {v: k for k, v in dic.items()}

while True:
    string = input("What would you like cypher?: ").upper()
    task = input("Would you like to encode or decode?: ").lower()
    while True:
        try:
            key = int(input("What key would you like to use?: "))
            break
        except ValueError:
            pass
    if task == "decode":
        key = -key

    list = (dic[x] for x in string)
    encodeNums = ((x + key) % 27 for x in list)
    encodeStr = (dic2[x] for x in encodeNums)
    print("".join(encodeStr))