1) What device are you using to visit this webpage? is it an android tablet? Apple iPad?
https://grapheneos.org/install
2) What web browser are you using for that website?
3) did you tap "build number" lots of times to enable "developer options? Then go to developer options to enable "OEM unlocking"?
4) did you hold down the volume down button while your phone turned on to get to the bootloader? See this picture as an example. It's okay if your phone does not have the exact same words and numbers
( https://www.androidauthority.com/wp-content/uploads/2018/10/Pixel-3-bootloader-unlocked-screen-1280w-720h.jpg )
5) usb-c on both ends of the cable is preferred. Make sure it is fully plugged in then do not bump or move the USB cable.
6) when you push "download release" on the website, a new menu should pop up asking to connect to your phone.
7) pushing "flash release" will take time. Wait until it changes to past tense "flashed"
Let me know the answers to the questions or any questions you have.
Do not use a usb hub. See this photo ( https://www.corporatearmor.com/wp-content/uploads/2022/10/HB30C4AB_1036857153_GYiWvPoPqI.jpg )