[Google Pixel 4 XL ROMs, Kernels, Recoveries, & Oth] GROOVYJOSHCLARK:
Wait hold on a sec, are you doing it from the web or local cmd ADB? Do not use the web version, follow the steps I laid out using local ADB with the flashall.bat FROM the FULL factory firmware but NOT from the web version. That's almost never going to work rolling back a version (from the web). This is why I asked for a pic of the error in my first reply along with your EXACT steps taken. This shows everything, and not how it should be done but would have saved many replies if I had the pic and steps. This is the issue if you're trying to do it from the web. I'm not even sure that will work (or even CAN work) because it's rolling BACK a firmware. This is also why I asked how you were exactly doing the process, something didn't line up and that's what it was. This may also be a really really really dumb question, but are you certain beyond a shadow of doubt, your bootloader is definitely unlocked? That would ALSO prevent a roll back.

I was also under the impression you were having that failure in the ROM install, NOT the firmware (and also why others said possible bad download). The web isn't going to work to roll back firmware.

Just to clear up a few confusing terms being used, your picture is firmware, not ROM. PixelDust is a ROM, not a firmware.

Download the firmware (FULL factory firmware) and save offline. Then follow my ADB steps a page or two back and flash the firmware to both slots, then try the ROM install again. You need to prepare the device properly to remove that 12 firmware and I don't think the web will do it. I know without question the offline version works with the firmware and ADB from my PC.

Now I understand why you didn't know what the -w option was in the flashall. You're not using that method but you need to or you may not ever get passed this error without an unlocked BL and using a FACTORY image (factory images are used to overwrite and restore the entire device BACK to how it was intended to leave the factory) so factory firmware images can and do restore everything back to how it should be but not an OTA. OTA's can only update the current installed firmware but cannot roll back. To roll back a firmware you must be BL unlocked with using a FULL factory firmware image using LOCAL ADB but not that web flash version. I don't know how the web version would handle rolling back a firmware without local ADB and factory image so use the local ADB and betcha it works this time and you get PD running (assuming BL is truly unlocked). We didn't discuss this piece (bootloader unlock), so it's assumed that's already done. Without an unlocked BL, you cannot do this.

Here is the direct link again to the offline firmware:

https://dl.google.com/dl/android/aosp/coral-rq2a.210305.006-factory-82bad63d.zip

If I'm understanding your process right, you're using that "FLASH" link on the left, but NOT downloading the firmware locally using the right side "LINK". Download the firmware then extract and make sure ADB is in your path. Switch to slot A, then run flashall and before it boots into the OS (very end right after it wipes userdata), interrupt the boot with volume down plus power (at the very second it reboots, but not before it finishes flashing, and before it boots into the OS). Then redo firmware on slot B, then when it finishes, allow it to boot that OS, go through quick setup and skip everything (no g account, no security, nada), then shutdown to ADB and install rom per the dev steps and assuming you did everything, and you're unlocked bootloader, you will be able to install and run PD11!

Do not take ANYTHING I said negativity, I am NOT attacking anything you said, did, or have done. I'm only explaining each step in that process to help anyone else who may need this so please don't take offense to anything written. I made a point of stating that picture comment twice because it would have helped tremendously if I saw that picture right from the getgo. Along with saving you time and wasted hours doing this so I was trying to point out the critical nature of missing pics, logs, or steps. It makes it more difficult to help when I don't have all of the information. I deal with this every day in my career with my clients and team that reports to me.