Try it again from the start redownload the SD card firmware and flash it again.

Check this guide below for the right procedure.

- https://support.bitmain.com/hc/en-us/articles/115000216034-L3-L3-and-L3-Control-Board-Program-Recovery

Sometimes the issue is that the card reader is full of dirt so cleaning them might solve your issue and sometimes the issue is a fake SD card so use authentic SD card and try to flash it again.