My team reported me that they are facing  issue while assigning quota for a particular Site Collection , They were encountering an error user not found.

This was happening as the owner of the site had been removed & new owner was not set.

To resolve this below command line is helpful.

But as Site quota was filled in our case so what I had done :- released some space out of recycle bin so that this command was successful.

This Should be run with a farm admin account on the sharepoint server.