Principal '' could not be found or this principal type is not supported.

Even I got the same error when I tried to create a user role for the slot using the below command:

CREATE USER [appname/slotname] FROM EXTERNAL PROVIDER;

So, I used the command below to fix the 'principal type is not supported' issue when creating a user role for slots:

CREATE USER [<AzureWebAppName>/slots/<SlotName>] FROM EXTERNAL PROVIDER;

Output: