def mul():
  return sum([i for i in range(1000) if i%3==0 or i%5==0])

Look at what the function range() does. Here you iterate from 0 up to 1000 included.
Re-read the instruction. Re-read your code, then you will find your mistake as your program logic isn't wrong.

I don't get where the issue is, if you change it during the kata it works just fine. Can't we view it as a way to learn how to debug?