I suspect your site still have Jomsocial files e.g. JoomlaFolder/administrator/components/com_community/libraries/core.php

This is why you hitting this error since your database no longer have this #__community_activities table.

If yes, perhaps you can try delete all the Jomsocial file or temporary rename this file and see how it goes.

If the issue still persists, can you update your FTP access at your first post so we can better have a check?

And which discussion post you would like to delete it so we can try to delete it from your site backend.

Hi again.

JomSocial has never been installed on the site. Only EasyDiscuss, EasyBlog, EasySocial and Easy Profile are installed. I've tried to delete the posts from backend but I get the error.

I will update my post with FTP informations.

That was strange, it might be someone install Jomsocial on your site before, because i saw those Jomsocial folder exist on your FTP server.

I already temporary renamed these following folder, it should work fine now.

JoomlaFolder/administrator/components/com_community_
JoomlaFolder/components/com_community_