SQL Server principal "dbo" does not exist,

USE [<dbname>]
GO
sp_changedbowner '<user>' -- you can use 'sa' as a quick fix in databases with SQL authentication

KB913423 - You cannot run a statement or a module that includes the EXECUTE AS clause after you restore a database in SQL Server 2005

I resolved this issue by setting database owner. My database did not have had any owner before this issue. Execute this command in your database to set owner to sysadmin account:

use [YourDatabaseName] EXEC sp_changedbowner 'sa'

After restoring a Database from SQL2016 to SQL2019, I had the same issue when I try to access Database Diagrams. I had the correct Database owner already but owner of Files was empty. Once I set that, it worked properly...

Do Graphically.

Database right click-->properties-->files-->select database owner-->select [sa]-- ok