If you are using MiFlash:

But what does "FDL2 failed" actually mean? Why does it happen? More importantly, how do you fix it without bricking your device?

error. Initial diagnostics suggest a possible mismatch between the PAC file and the device's CPU revision, or a driver timeout. I will attempt to re-flash using a high-quality data cable and verified firmware to resolve the handshake failure."

: Takes over from FDL1 to manage the actual writing of firmware partitions (like system, boot, and recovery) to the internal storage. Common Causes of FDL2 Failure