Best way to remove wpForo
Please copy the error messages and leave those here to allow us check the conflict point. Maybe this is a server incompatibility issue. Make sure your server matches wpForo requirements: https://wpforo.com/docs/root/wpforo-installation/server-requirements/
wpForo has an Uninstall button on Plugin page. You can click on this button to remove all data. Then click on delete button to remove wpForo files.