Do I understand you correctly: You have GrapheneOS installed on your Google Pixel device and want to go back to Stock Android? I recommend using the Web Installer from Google to do it. First you need to remove the non-stock key. See here for the official manual.
I assume you have the fastboot drivers installed? What OS are you using?
Another common cause for errors is the cable. If possible, use the original cable that came with your Pixel device, otherwise try different ones (short and thick USB cables that come with external SSDs always worked for me).