[sql-server] Cannot use special principal dbo: Error 15405

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

"Cannot use special principal dbo"

Server roles of the user:

To fix this, open the SQL Server Management Studio and click New Query. Then type:

USE mydatabase
exec sp_changedbowner 'sa', 'true'

This answer doesn't help for SQL databases where SharePoint is connected. db_securityadmin is required for the configuration databases. In order to add db_securityadmin, you will need to change the owner of the database to an administrative account. You can use that account just for dbo roles.

Fix: Cannot use the special principal ‘sa’. Microsoft SQL Server, Error: 15405

When importing a database in your SQL instance you would find yourself with Cannot use the special principal 'sa'. Microsoft SQL Server, Error: 15405 popping out when setting the sa user as the DBO of the database. To fix this, Open SQL Management Studio and Click New Query. Type:

USE mydatabase
exec sp_changedbowner 'sa', 'true'

Close the new query and after viewing the security of the sa, you will find that that sa is the DBO of the database. (14444)

Source: http://www.noelpulis.com/fix-cannot-use-the-special-principal-sa-microsoft-sql-server-error-15405/