You should be able to see the full error message in either the Flarum log at <flarum>/storage/logs (if it's a PHP error) or the browser development tools console (if it's a javascript error)
You can remove an extension with composer remove <package name>.
If you just want to disable it without uninstalling, you need to edit the enabled_extensions setting in the settings table of the database and remove the extension ID from the array of enabled extensions.
