DanMentzel If you're in incognito mode, go back to the normal mode. If you're using Brave and shields are "up" or "on" or whatever they call it, turn that feature off for the GrapheneOS website. Also, make sure you have enough free space on the device you're flashing from.
Those are pretty much the most common causes for the error you're getting.
If you want faster, real time help, you can ask on Discord/Matrix/Telegram. Information for those chat rooms is here: https://grapheneos.org/contact#community