Is there an error message in server.log?
On which os do you have the problem? In windows sometimes files can be locked prventing delition. Then try after a reboot. You can also always delete the config file in STORAGE_DIR/configs and delete printer data in STORAGE_DIR/printers/<SLugname> when server is down. That is more or less what delete printer does anyway, just when deleting fails it would not delete it - or if it sees a klipper installation with it's slug name regardless of the fact you switched it to marlin.
