I had the same issue too… I've got a pixel now, I had my 5X laying for years, but just recently I've tried charging it, and got the red LED blinking indicating the power is too low, so I've tried pressing pwr button & vol down, it boot into fastboot & got error saying my device is not reliable & got into TWRP, check the /system/ folder but got nothing inside.. shut it down while keeping it charged, downloaded ROM, Tied boot into fastboot again but No luck & now cannot get into TWRP again.. I guess have to wait the phone get fully discharge now & try it again.