if you added all the values into a, you would get the sum in a. so what then would sum1 be?
(clearly nothing since it will not run, but as you are trying to use it, that doesn’t fully make sense)
and then if you removed sum1 you’d have next contradiction, you’d be creating a list with a list comprehension, but then not using that list in any way
syntactically, you’re using a statement in place of an expression, similar to how this is not allowed:
1 + for n in range(3): n