You should use assignment “=” not Set.
It’s kind of hard to see what you’re trying to do here. It looks like your Checkpoint01, etc are not defined… generally trying to access persistent objects in c++ isnt a good idea. Try creating a function where you pass it an object and it does the procedure, something like:
UFUNCTION(Blueprintcallable)
void AddCheckpoint(AActor* checkpointactorref);
UFUNCTION(Blueprintcallable)
FVector GetCheckpoint();
Call these from blueprints, then work backwards from there. Start with something super simple (maybe using dummy info) then build from that.
Read over this on how to integrate with BPs :