Hi Azam
This error is due to other TU has assigned to door you want to assign your above mentioned TU.
resolution >> check /scwm/mon to find TU and door assignment.
checkout the TU which has done unload and free the door for next TU.
if you are doing only for testing purpose, you can also create new door, door bin and all, in customization and use in above scenario.
As you unsigned door or create new door in both the door can be used for your above process.
I hope this should resolve your issue.
Regards
Suraj