What exactly your miner unit model? is it an antminer s9, s9i, s9j or other Antminer model?
You must provide more info about the miner like the unit model, kernel logs or screenshot of the status board of the miner so that we can easily find if why you always get "signature missing"
It seems you are trying to flash with a wrong or incompatible firmware? You must flash the miner with a correct firmware in order to flash the miner successfully or maybe you flashed the miner with moded firmware from an untrusted source?
If I'm right the only thing comes to my mind that may fix this kind of issue is to flash the control board with SD card you can follow the unbricking method from here
https://bitcointalk.org/index.php?topic=2386296.0