Probably because you’re referencing the pawn class instead of referencing your pawn as an instance.
Just like in unrealscript, PlayerController derives from Controller. So look in controller.h and you will see a member variable called “Pawn” of type APawn*. So you use that to access FaceRotation.
TLDR? In your custom playercontroller, try this instead:
Pawn->FaceRotation();