You put the wrong version of TWRP on your tablet probably? when you restart and it boots into TWRP, on the starting screen you should be able to use fastboot.
---------- Post added at 04:33 PM ---------- Previous post was at 04:31 PM ----------
When it happened to me, it was a bootloader and TWRP conflict: I flashed the jb version instead of the -42 Version. to fix it, I did fastboot reboot bootloader and then got back into android and flashed the correct one from Terminal Emulator. Or you could just flash it from there.
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