Hi,
I actually flashed the OP7T, but hope my experience is also applicable to 8T.
Perhaps a stupid question, but did you follow the steps exactly, namely those (text in () brackets are my comments):
7 - Sideload the /e/ .zip package.
On the device in TWRP, (I think it should say e-revocery, not TWRP, but I might be wrong about the 8T recovery version)
select “Advanced”, “ADB Sideload”, then swipe to begin sideload.
(for 7T it is “Apply update”, “Apply from ADB” instead)
On the host machine, in the console , start the sideload of the package by typing
adb sideload filename.zip
The error you posted is exactly what I get if I run the adb command without starting the sideload in recovery first.
BTW, is the 8T recovery really TWRP, or does it have the /e/ logo? I think the docs might be slightly outdated