TC75: signature verification failed, installation aborted



I have 3 brand new TC75 non-GMS (production units purchased from Scansource) that I am unable to convert to GMS ones or even flash their own non-GMS software. I always get the message in recovery (E:footer is wrong, E:signature verification failed, installation aborted). I have repeated the download several times (in case my file is corrupt), used internal storage instead of external SD but nothing changes. The images are from our support portal. My units are on 1404 non-GMS image and wish to convert them to the respective GMS one. Any ideas what could be w


Thanks a lot for any input.