Failed to validate system image

Hi guys, a friend of mine had a problem with his Moto E XT1021 and I'm trying to help him. I'm stuck on this screen and i don't know what to do:

AP Fastboot Flash Mode (S)
...
Battery OK
Devis is LOCKED. Status Code: 0
Connect USB Data Cable

...

failet to validate system image
Fastboot Reason: Fall-through from not charger-boot mode


He told me that he started to have problems after updating his phone. Whatever I do, I get:

failed to validate system image
Boot up failed

I tried to unlock the bootloader, but when I type fastboot oem get_unlock_data I get this error:

(bootloader) Could not get unlock data! OKAY [ 0.166s] finished. total time: 0.173s



...any suggestions? Thanks.

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