DeletedUser353 I see you asked this question an hour ago. While people on the forum can try to help, you'd probably get faster help in the chat rooms. For help with flashing, you can ask in the "General" room/channel. https://grapheneos.org/contact#community-chat
I'd say that if it worked once and isn't working now, maybe you have too many tabs open, or some other app or system process is blocking/holding the usb connection somehow? You can try restarting the browser and/or computer and trying again. Make sure you're not in incognito mode and only have one tab open. There've been times in the past where extensions have caused issues. I don't think it's likely in this case since you already unlocked the bootloader, but worth mentioning. You may also need to stop the fwupd service.