I'm still stuck here, too . . .Hello! I need a bit of help... I have a TB-606F, 4GB/64GB version
I'm stuck on step 5. I've unlocked the bootloader, flashing TWRP and vbmeta seems successful, but then I cannot reboot into recovery. Using the fastboot reboot command I get "fastboot: usage: unknown reboot target recovery". If I try to reboot using volume up + volume down + power it eventually boots to Android.
Any thoughts are much appreciated!