Cannot flash twrp-3.2.3-0-cedric.img (Motorola Moto G5) ("Image not signed or corrupt")
The locked bootloader doesn’t let unsigned images to be flashed. So, the Image not signed... error indicates that there is a possibility of the bootloader not being unlocked.


Make sure the bootloader is unlocked.
Make sure you are using right version of TWRP for your device.
Try fastboot boot /path/to/twrp.img instead of flashing. If it boots, flash the recovery image from within the recovery. Afterwards, any zip should be successfully flashed (custom ROM, GApps, mods etc.).