Does a computation tree for a probabilistic Turing machine have to be a binary tree?
There are many ways to think of how a probabilistic Turing machine works.  One way is to imagine that it has the ability to flip a fair coin (sample a random bit) at any point.  In that case, it's reasonable to imagine a binary tree, where each branch corresponds to a coin flip -- the fact that we are choosing a random bit / flipping a 2-sided coin is why it is a binary tree.  Another way to think of a probabilistic Turing machine is that it has an additional input tape that is full of random bits.  There you could imagine a binary tree, which each branch corresponds to another bit on that additional input tape.

Typically probabilistic Turing machines can sample a random bit, i.e., a random number from the set {0,1}.  If you want a random number from a different range, you must emulate that by repeatedly sampling random bits and doing something with them.