Hi,
by standard, SAP prohibits item (or transaction) deletion if a follow up has been created from that item (transaction). `In order for the delete to work, the follow up item needs to be deleted first.
PS : pay attention at the status of the original transaction, if used as Quotation, a Release status transaction will be triggered when deleting the followup. -> you need to check the ECs that are called.
Regards,
Andrei