Graph unit comes pre-installed on supported systems (subset of all FPC supported systems). I don't really remember what major platform is missing, but it's either x86_64-win64 or x86_64-linux. Depending on your OS, there might be replacement units that has compatible interface so you just need to change the name in the uses clause. Look at your installation directory under units/graph subdirectory. It depends on how you get FPC installed, though. I have a feeling that you actually use Turbo Pascal instead of Free Pascal, Free Pascal would never try to look for .tpu files when resolving uses clause.