When we are going to design an Entity Relationship Diagram 1st time using SSMS in SQL Server, we will face an error of authorization like below.
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.
ALTER AUTHORIZATION ON DATABASE::<db_name> TO sa