Class forward declaration not working with UFUNCTION

I tried building, rebuilding and cleaning the whole solution and just the project, the same error popping up each time. I shrugged it as irrelevant, but perhaps the other two errors might have to do with it or lead to a solution?

UnrealHeaderTool failed for target 'BattleTanksEditor' (platform: Win64, module info: D:\Users\<USER>\Documents\Unreal Projects\UnrealCourseSection5\BattleTanks\Intermediate\Build\Win64\BattleTanksEditor\Development\BattleTanksEditor.uhtmanifest, exit code: OtherCompilationError (5)).	BattleTanks	D:\Users\<USER>\Documents\Unreal Projects\UnrealCourseSection5\BattleTanks\Intermediate\ProjectFiles\

EXEC	1	


Error	MSB3075	The command ""C:\Program Files\Epic Games\UE_4.15\Engine\Build\BatchFiles\Build.bat" BattleTanksEditor Win64 Development "D:\Users\<USER>\Documents\Unreal Projects\UnrealCourseSection5\BattleTanks\BattleTanks.uproject" -waitmutex" exited with code 5. Please verify that you have sufficient rights to run this command.	BattleTanks	C:\Program Files (x86)\MSBuild\Microsoft.Cpp\v4.0\V140\Microsoft.MakeFile.Targets	41