This is a common issue right now if you flash the miner already with the latest version(2019 firmware) and then you are trying to downgrade you can see this error "cannot find Signature" there's a method to bypass this error that you can find here but the problem the 2019 firmware disabled SSH access.

So the only solution that I think might solve your issue is to flash the control board through SD card. Recently Bitmain decided to release a method flashing S9s through SD card.

If you don't know how to do it you can follow it from this link below.

- S9 series (S9, S9i, S9j, S9 Hydro) Control Board Program Recovery.