This is a Hardbrick and it's the hardest brick to fix, most of people fail or just forget about doing it because it require opening the tablet and having access to the motherboard and then soldering some wires to it with a SD Card reader to get access to the eMMC directly and trying to flash back the boot partition which is encrypted by Amazon... Some says that they successfully fixed it on forums but most don't, I have one in the same situation as yours and I didn't try to fix it yet as I didn't find the required files to flash them.
if you are still interested by fixing I will send you the links later so you can know more about the process.