It looks you have to remove the back cover to disconnect the battery before executing "sudo ./bootrom-step-minimal.sh".
See my post https://forum.xda-developers.com/t/...hd-10-2017-suez.3913639/page-75#post-86690371 and the 2 following posts...
I have two kinds of these Fire tablets, the 7 inch ones and the 10 inch one.
On the 7 inch ones I would have to take the back off and ground out the clk pin I think it was while executing the script, you need 3 hands. Anyway I managed to do those fine.
The 10 inch ones I realized you don't, you run the Waiting for bootrom command, and literally hold the power button down on the tablet for a good 5-10 seconds and the command starts running. No need to take the back off, plus I think on the 10 inch tablets there are ground shields around everything makes it really messy unless you can hot air them off or something.
Try just holding the power button on the 10 inch tab, while the script on the shell says "waiting for boot rom" I think a lot of these guides could possibly use some extra verbiage when it comes to how to's but that's just me.
Good luck, hopefully that's all it is.