Hello Stonewall40, If you try & clean the drive 2 times in a row & it can’t get the heads clean then the heads then the heads of the drive will need to be replaced.  Now if the system is still under warranty then I would open a support case and they can get it replaced for you.  If the system is not under warranty then what you can do is do a search for a replacement drive & you can replace just the drive in the PV124T if all other hardware is ok, or you can do a search for the whole replacement unit.   Please let us know if you have any other questions.
Greetings! I have a customer's PowerVault 124t is getting the clean head message. They run new cleaning tapes but still will not run a backup.   Will the system need new heads by chance? Wondering if there is any way to repair the unit with new heads, or simply swap the machine out with an refurb unit.   Thanks for the help!  
124t not cleaning