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

What is the initial value of min in this example?

You can replace

(1 until number).map(_.toLong)
with
(1L until number)

Scala will infer that the range should be of type long by the type of the start of the range. I'm pretty certain that this should be more efficient as it should just create the range once as the correct type, rather than create one Int range, and then one Long range.

You could also change
if(a%3==0||a%5==0)true else false
to
(a%3==0||a%5==0)
Both do the same job, but the second is clearer and should be more efficient because it doesn't need to use a bool to look up a bool.