Hi @Perez ,
Is the user a guest user?
I have made some test, I found that when testing with Guest user, who is also in my organization Azure AD, the error message “Reference user or group is not found” will be returned.
In the following test, I am using a person or group column value in the Assigned User Ids field. When it is an organization user, it works successfully. When it is a guest user, it failed.

Best regards,
Mabel