Smoth Scroll not work

Hi,
I am writing to you because "smoth-scroll" function in my website is not working. I performed this procedure:

1) I have enabled "smoth scroll" from the settings.
2) I inserted the "smoth-scroll" class on the menu
3) I inserted the snippet for the duration with the "Code Snippet" plugin.

I also disabled Siteground Optimizer since I read on the forum that it might create conflicts.

I don't understand what problem is created.
Thanks in advance for any suggestions.

Stefano