This was caused by a BT outage late last week, which affected a number of Royal Mail services, including the manifest processing service. As a result, the manifest that was filed on the 19th of May became stuck in your system with a failed status and has not processed since.
I have just manually processed this manifest for you and you should now be able to continue working through your OBA consignments.
Apologies for the delay, had to confirm this with our shipping team.
With regards to the manifest that was filed on Friday, indeed, you could get in touch with Royal Mail and have them cancel it for you. You would need the Sales Order ID, which you could obtain from the OBA website.
The one that is on your account at the moment will need to have all of it's consignments cancelled the way that you described, however, this will in no way affect the status of the tracking number on your selling channels, nor will it affect the tracking that was received by the customer.