I can get to the game menu, but when I hit “Play”, the game says the disc needs to be inserted. I still have the ISO on my system.
Some games require you to have the disc inserted to play it.
Either is because the game does not copy its resources to the hard drive and it loads them from disc (like many Windows 95/98 games), or is an antipiracy protection.
You could try to mount your ISO on CDEmu and install your game from there.