Download the game again.
This should be directed to itch.io support. Developers do not control available payment methods.
Download the new version when it comes out or use the itch.io app.
Update your graphics drivers, or add --rendering-driver opengl3 to your game's launch options (if not using the itch app, open the app from the command line with said arguments).
When I try to download the Linux demo, the google drive link says I don’t have access.
RetroArch is that app, the cores are the emulators it uses.
Retroarch cores are the emulator bits of retroarch. You'll want both.
