UnrealHeaderTool issues with RPCs

In my header file for a pawn I’m attempting to declare an RPC.
The following compiles properly:

UFUNCTION( ) void ClientSetPosition(FTransform position);

Yet this does not compile:

`UFUNCTION( Client )

void ClientSetPosition(FTransform position);`

I Get the following error:

UnrealHeaderTool failed for target 'MultiEditor' (platform: Win64, module info: C:\Users\myself\Documents\Unreal Projects\Multi\Intermediate\Build\Win64\MultiEditor\Development\MultiEditor.uhtmanifest, exit code: OtherCompilationError (5)).
The command ""C:\Program Files\Epic Games\UE_4.21\Engine\Build\BatchFiles\Build.bat" MultiEditor Win64 Development "C:\Users\myself\Documents\Unreal Projects\Multi\Multi.uproject" -WaitMutex -FromMsBuild" exited with code 5. Please verify that you have sufficient rights to run this command.

Any thoughts on why this might be the case?