Gatsu clicking "Uninstall" in the admin panel is the only way to delete the extension data, if that's what you want to do. It will remove the tables from the database, effectively deleting all of the data created by the extension, including terms accept dates for all users.
I have just tested the extension again on beta 13 but don't see any issue. Maybe there's some kind of extension conflict on your forum.
Can you share a screenshot of the full error message you get ? Does it appear in a red modal inside Flarum?