I added them. What's the issue?
There are two aspects of this that might cause an error.
First, make sure that you are following the proper naming conventions for variables. I had a student once use capital X and Y - but that's not right! Proper conventions call for underscore_case, which uses lowercase variables (x and y).
I hear thoughts: "That doesn't matter!" But it does! When you're building a complicated program with multiple people coding, naming conventions quickly become super important. Learn them now so your future coworkers don't toilet paper your house.
Second, return the value directly. If you have it coded like this:
def sum_two(x,y):
sum = x + y
return sum
you instead need it to skip the variable name on line 2. There is no need for it; you're just making your program unnecessarily longer. You can do the addition and the return on a single line.