Helpless_user Please connect your phone and ensure that you have the platform tools and can open a CLI and type fastboot devices to see if the device is listed. If not you require the USB driver and/or a fully USB standard compliant cable and port.
Don't forget Linux Mint is not a supported OS, it can be tried and you can see if it works though.
As you're using it unsupported, have you also read fully the rest of the instructions? What browser are you using etc?
Also the forum is not the most dynamic resource for rapid response support for installation, our [Matrix] Community Space would be better: https://matrix.to/#/#community:grapheneos.org