Problem
SQL Server has a broken reference somewhere preventing you from deleting a maintenance job, presumably a job that you created as a maintenance plan.
OR (worse yet SQL claims that it isn't "aware" of this job)
Solution
So the ultimate solution to this problem is to delete the job and all of its references/dependencies/children using a stored procedure (don't worry, I provided the stored procedure below).
Step 1
Backup your msdb database.
Step 2
Select the sysjobs from ...