It seems you've set 90% width to forum content and the sidebar doesn't have enough place. Please remove all widgets from wpForo Sidebar in Dashboard > Appearance > Widgets admin page and set 100% the width instead of 90%. Or remove the custom CSS code you have for:

#wpforo #wpforo-wrap .wpforo-content {

Thanks, I made it smaller.
And it works now.

You should remove the width parameter for that custom CSS. wpForo works well without that CSS. This css may make problems in mobile.

Thank you.
I had received this CSS from someone from this site.
Works well now