The “Path to Free Pascal Compiler executable” should be the full filename to the FPC executable, ending like fpc.exe (on Windows). You specified there only the directory, ending with ...x86_64-win64\. Looks like you should just add fpc.exe.
The hint visible at the screenshot above tried to communicate this, but I recognize this was confusing, it said “Path to the Free Pascal Compiler executable…” (which could be interpreted as filename or directory; and in fact right below, for FPCDIR, we say “path” to mean directory…). I improved it now, to say this:
Note: You could also just get CGE from Download | Castle Game Engine , which has a bundled FPC version, and then the bundled FPC would be used automatically.