Received a debt recovery letter. Never received no parking tickets

Received a debt recovery letter for Heathrow drop off. Never got the initial pcn. What are my options in appealing or getting it back to the first stage

Little or none.
Please show us. https://www.ftla.uk/private-parking-tickets/read-this-first-private-parking-charges-forum-guide/
APCOA?
They don't take people to court so you can ignore if so.
Debt collectors have no power and should be ignored anyway.

If you didn't get the original PCN, are you the registered keeper of your car and is its V5C correct?

If you want to see the original PCN you can submit a Subject Access Request to whoever it is (not the debt collector, the parking company they represent).