fastboot flashing error: "Partition should be flashed in fastbootd"
I'm sure you figured it out by now but for any future users searching for a solution, I had the exact same issue on my Pixel 3a XL. You need the most up to date version of ADB and FASTBOOT. Grab it from Google to be sure you have the most recent, you only need the SDK Platform-Tools. Don't have to download the whole SDK Platform