Cannot use special principal dbo

Error: Cannot use special principal dbo: Error 15405

Today I got a call where user asked that all required roles were not assigned to user database after UAT refresh.

User was having "db_owner" but when I tried to assigning him other roles created for specific purpose we get below error  "Cannot use special principal DBO"

 I tried to give other permissions to user in the User Mapping section of a database. But, I am encountering this error.

On investigating I found:

This is happening because the user is the actual owner of the database  - as such, they can only have db_owner, and cannot be assigned any further database roles. Nor do they need to be. If they're the DB owner, they already have permission to do anything they want to within this database.

 To see the owner of the database, open the properties of the database. The Owner is listed on the general tab. To change the owner of the database, you can use

1. sp_changedbowner

2. Alter Authorization

3. From GUI : goto database properties , and change as below screenshot

Changing owner of database to sa , will allow you to assign any role to your user