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.
Workaround / Fix / Solution :
Here are the steps to solve.
-- Replace YourDatabaseName in following script
ALTER AUTHORIZATION ON DATABASE::YourDatabaseName TO sa
- Select your database >> Right Click >> Select Properties
- Select FILE in left side of page
- In the OWNER box, select button which has three dots (…) in it
- Now select user ‘sa’ or NT AUTHORITY\SYSTEM and click OK.