Database diagram support objects cannot be installed because this database does not have a valid owner.

This sometimes happen when you attach a database from a different machine or after restoring a database from scratch

Solution:

EXEC sp_dbcmptlevel '<<DBNAME>>', '90';
go
ALTER AUTHORIZATION ON DATABASE::<<DBNAME>> TO "<<LOGINNAME>>"
go
use <<DBNAME>>
go
EXECUTE AS USER = N'dbo' REVERT
go