dm-verity corruption and "your device is corrupt" on Alcatel device

A friend gave me an Alcatel phone to fix it, the phone updated automatically and when it starts up, this happens: > dm-verity corruption > > Your device is corrupt. > It can't be trusted and may not work properly. > Press power button to continue. > Or, device will power of in 5s 5 seconds after the message is displayed, it restarts and displays the same message. I tried putting it into fastboot to do what they tell me to do with this error, but the phone reboots by disconnecting from the PC. Even to put it in fastboot, I have to use the volume up arrow and the power button, I choose the fastboot option, and when I choose fastboot, the phone does not show a terminal like they do in the tutorial, it seems as if it was turned off. I connect it to the PC and the same thing happens, it restarts every 5 seconds and adb does not recognize it. I do not know what to do.