When I right click on the database diagrams folder in SSMS to create new diagram. Following popup window open with error.
From the Message you can clearly see that the database does not have a valid user. We can have a valid user by running a query or through SSMS.
ALTER AUTHORIZATION ON DATABASE::TestDB TO sa
Change Database name to your database in query.
Go to properties of a Database by right clicking the Database and then select File.
Change the owner by clicking the dotted button in highlighted part in the image below.