MTA says you have a trainer but you don't
Try closing every other open window first. If that doesn't work, try uninstalling, then reinstalling MTA 0.5 again. It has also been reported that changing game version from 1.0 to 1.1 (Vice City) can fix this, though we're not sure why - we're looking into it. Further Update: All the error reports that have come in about this are using Vice City 1.0. Try updating to 1.1, if this _doesn't_ work, contact me.