Check the logs in /var/discourse/shared/standalone/logs/rails/production.log and see what the error is and report that.
And then you can go into rails and un-do the changes you made. I think with something like
Sitesettings.selectable_avatar_whatever=false
You can use tab completion after you type selectable or maybe start with avatar? I don’t know how that stuff works off the top of my head.