For some reason Microsoft decided not to enable that function in some editions of RTM in SQL server 2008 (annoying but meh). So looking around
Category: Maintenance Plans
Can’t Modify/Delete Maintenance Plans in SQL 2005 after Rename
Credit: Tracy (SQL Server Central, Link) After renaming SQL Server 2005 server the maintenance plans couldn’t be deleted. When trying to delete or modify a
SQL Canada (SQLCAN) 