@Luna Bar How are you trying to install the stock rom? Are you flashing stock firmware or trying to install a rom.zip through the stock recovery? If the latter, you cannot install a rom through the stock recovery.
I have no ZTE experience, so I can't give too much advice, but I would think you need to install the latest version of TWRP back to your device and then flash a new rom or flash a stock rom.
Or if stock firmware is available for your device (not sure how they flash stock firmware on a ZTE) you might want to download the official firmware and use whatever tool ZTE requires to return the device to stock.