Hello!
I assume you press "Run" (F9) from Castle Game Engine editor?
1. Make sure your FPC is detected by Castle Game Engine editor -- go to "Preferences" and then "FPC and Lazarus". It should show your FPC (either explicitly configured or auto-detected) in "FPC Path".
2. If you have FPC installed, make sure it is FPC compiler for x86_64. In particular, if you use FPC installed manually from https://sourceforge.net/projects/freepascal/files/Win32/3.2.2/ , note that this is a compiler for Win32 (i.e. 32-bit version of Windows), due to rather historical/internal reasons. You want to install version for Windows 64-bit: e.g. by adding there `fpc-3.2.2.i386-win32.cross.x86_64-win64.exe` from https://sourceforge.net/projects/freepascal/files/Win32/3.2.2/ . Or install Lazarus and FPC together from https://www.lazarus-ide.org/, Lazarus installers for Windows come with FPC for Windows 64-bit.
3. Most advised: Note that new Castle Game Engine versions actually already include a proper FPC version, for both Linux and Windows (that is able to compile for Win64). This FPC versions is also auto-detected by CGE by default.
To try this, download Castle Game Engine from our website ( https://castle-engine.io/download ) and not itch.io. This itch.io page features a bit older version, 7.0-alpha.2. We're working now on 7.0-alpha.3 and it already features a lot of new features and improvements, the "bundled FPC" being just one of them, see https://castle-engine.io/new_features_in_castle_game_engine_7.0 . Our https://castle-engine.io/download allows to download version 7.0-alpha.snapshot, which is our latest version that is soon going to be promoted to 7.0-alpha.3.
If none of this helps, I will need a bit more information about your setup -- what is your OS (Windows, Linux?), how did you install Castle Game Engine (from this itch.io page, from https://castle-engine.io/download ), how did you install FPC. We can continue this thread here, you're also welcome to create a thread on our forum ( https://forum.castle-engine.io/ ) or make a GitHub issue. See https://castle-engine.io/talk.php for various places where to catch me :)
Regards,
Michalis