People must be having this issue all over the world!
You tried to create or alter a table column (or likewise attributes in a table) with Microsoft SQL Server Management Studio and get the following error.
ERROR: Cannot use the special principal 'sa'. Microsoft SQL Server, Error: 15405
You can get into the SQL server with 'sa' in Mixed Mode authentication. You have even given the sa permissions to all of the databases.
The database ownership is still not correct when you restored or re-attached database. This happens for example, if you restore a database using an integrated authentication account.
Try This Fix:
Open the SQL Query windows in the Microsoft SQL Server Management Studio, and try something that resemble below by changing. TheNameOfYourDatabase to the database you are using. Do this with every database that you have that you need the 'sa' access. This will change the owner of the database to the account you designate.
exec sp_changedbowner 'sa', 'true'