Luckily, Rama explains the simple solution to this issue on the UE4 wiki page about BlueprintNativeEvent functions.
Check out the section way down at the bottom called “Overloaded function not found…”
Basically, the way I understand the issue is, during compilation the Unreal Header Tool sneakily generates a signature for your BlueprintNativeEvent function that passes your function parameters by constant reference instead of simply by value. In other words, the UHT sneaks in a “const” and an ampersand where us well-meaning programmers don’t expect 'em to be.
Thankfully, the fix is easy: just change your function signature to pass the FString parameter by const reference instead, which will cause it to match what the UHT mandates. Specifically, try changing your function declaration to:
void AProjectileBase::FiringSetup(const FString& directionFace)
Finally, based on your screenshot, it looks like you’re likely to encounter another problem that I just wrestled with yesterday:
When defining your BlueprintNativeEvent function in your .cpp file, you must add the _Implementation suffix to the function name, or else you’ll get compiler errors regarding duplicate declarations or some such. Rama explains this issue also on that same Wiki page.
Hopefully, with that, things make slightly more sense and you can get your code compiling again. (I know I was shaking my head yesterday.)