The error happens because we scan for filenames as well as UClasses, so if the add fails because the source files were added but then later failed to build, we’ll still pick up that there’s a file with that name and prevent you creating another class with that name.
It shouldn’t be failing to find that header though… I can see that the .cpp file was in “Source\ShooterGame\Private\Weapons\ShooterWeapon_Instant_FU.cpp”, but where was the corresponding .h file?