Head on over to the Matrix chatroom over here and they'll help guide you through it (you need to create an account if you do not have one).
But it is almost always the user not quite managing to follow the guide, so please go and double-check that you haven't missed something here.
The most common problem is the USB cable or port:
Whenever possible, use the high quality standards compliant USB-C cable packaged with the device. If your computer doesn't have any USB-C ports, you'll need a high quality USB-C to USB-A cable. You should avoid using a USB hub such as the front panel on a desktop computer case. Connect directly to a rear port on a desktop or the ports on a laptop. Many widely distributed USB cables and hubs are broken and are the most common source of issues for installing GrapheneOS.