Hi Simple, I've noted a couple of issues you were having below to troubleshoot, you are using a Google Pixel phone that is compatible and supported yes?
Next are you using the official instructions or some third party youtube video?
Are you attempting to flash with the CLI or Web based installer method?
GrapheneOS has 99%+ app compatibility with the sandboxed Play Services and per app toggles to disable where necessary or desired our hardening on/off.
Banking apps rest in that 1% in some cases due to their reliance on deprecated safetynet or Play Integrity API. However as linked below we have a resource provided by the community for the community based on it's feedback.
Yes using other non stock cables can cause issues as not all USB C cables either C-C or A-C are fully standard compliant resulting in them not being usable for low level fastbootd (this is not the standard fastboot in the bootloader).
Use the stock cable, ideally with another Android device (no driver required), if using a laptop be aware some use a daughterboard acting as a hub internally for USB C dependent on how old it is, the same for front IO on desktops.
Also: