Database diagram support objects cannot be installed because this database does not have a valid owner. To continue, first use the Files page of the Database Properties dialog box or the ALTER AUTHORIZATION statement to set the database owner to a valid login, then add the database diagram support objects.

You may get the above error when trying to open the database diagram or trying to create a new diagram.

Solution

ALTER AUTHORIZATION ON DATABASE::DTLDW TO sa

or

Assign the Database owner to a valid login by right clicking on the database and choose properties > file.