I unlocked the bootloader of my Teclast M40 following this guide. I was also able to apply root.
Now I would like to flash a different rom but it seems as if anything I try to do as a terminal is not granted to me because I don't have the permissions.
Yet the bootloader is correctly unlocked and if I try to run your guide again it tells me that it is already unlocked.
I have tried both from Windows and Ubuntu, with and without platform-tools but I always encounter the same problem ...