[HELP] Can't flash TWRP Recovery

I Flash TWRP & CM 12 earlier everything works well...but one day I back to stock ROM which also locked the bootloader. Then I try to unlock the bootloader with earlier unlock code but it giving error like

(bootloader) Failed to erase partition
(bootloader) General unlock failure!
(bootloader) OEM unlock failure
FAILED (remote failure)
finished. total time: 10.212s