Booting into twrp unsuccessful

Moto devices with a stock ROM have a facility to replace the recovery with the stock image, TWRP handles this but you have to reboot into recovery once before starting the phone normally. Flash TWRP with fastboot and boot into it, allow changes (or first, say No, then do a backup!) then within TWRP do a Reboot -> Recovery, this will "set" the recovery so it will not be replaced.

fastboot boot twrp.img (replace twrp.img with the actual filename for your TWRP image)

TWRP should load. Do not allow changes and perform a full backup (keep this safe so you can restore to known good point if needed at some time). Once this is done, go to Home - Reboot - Bootloader

Flash TWRP with fastboot flash recovery twrp.img and then reboot into the bootloader with fastboot reboot-bootloader and start Recovery, TWRP should start.

In TWRP, allow changes and go to Reboot and select Recovery, the device should reboot and TWRP will start again. Select the option to allow changes, go to Reboot and select Recovery, this will "set" TWRP as the static recovery image.

You can now Reboot - System and the phone should start with TWRP left in place.