The sum method will return an Int which can cause problems for large values.
Here foldLeft is used to convert the values to long format as they are sumed.

thanks for the advices! Im a begginer on this language and its always good to learn new ways to do the same

In case of reduce, initial value is the first element of the list. For puprose of this kata we can assume list is not empty so initial value is not needed. Take a look at kata details.

I've tried other options too:
https://www.codewars.com/kata/reviews/5a6bb48a07365e9dad000005/groups/5ac759fa52763ee0030025a4
https://www.codewars.com/kata/reviews/5a6bb48a07365e9dad000005/groups/5a6c62d7512a62e44300060b
This one is the fastest one-liner. The first one is probably slower because it's filtering Longs instead of Ints, the second one - because of an extra map, although I'm not entirely sure about the reasons.
But this is still significantly faster:
https://www.codewars.com/kata/reviews/5a6bb48a07365e9dad000005/groups/5a6c66fb3a8fbdad4c000157