htc 626 metro pcs unlock problem

i have one htc 626s metro pcs locked on 6.0.1 i am trying to downgrade it but xtc not allowing me to downgrade just check it

Starting flash process...
target rom version: 1.23.1550.3
found large-zip header, file count: 16
processing file 1/16...
sending 'zip' (62666 KB)...
OKAY [ 5.435s]
writing 'zip'...
(bootloader) HOSD CL#774853
(bootloader) ERR [SD_UPDATE_ERR] Main version NOT ALLOWED
(bootloader) ERR [SD_UPDATE_ERR] sddownload failed: 19
(bootloader) ERR Update zip file failed.
FAILED (remote: 90 hboot pre-update! please flush image again immediately )
finished. total time: 10.558s
preupdate done, rebooting...
Phone DISconnected
Phone connected [FA5BSB005721] *** Fastboot mode
resending command...
sending 'zip' (62666 KB)...
OKAY [ 6.299s]
writing 'zip'...
(bootloader) HOSD CL#774853
(bootloader) ERR [SD_UPDATE_ERR] Main version NOT ALLOWED
(bootloader) ERR [SD_UPDATE_ERR] sddownload failed: 19
(bootloader) ERR Update zip file failed.
(bootloader) Update zip file OK
(bootloader) ERR ZIP ERROR CODE, 19
(bootloader) end@Done
FAILED (remote: 19 RU_MAIN_VER_FAIL os-version in android-info missing or i)
finished. total time: 12.459s
HBoot pre-update sequence
Flash process completed.
Flash not successfull, reboot will not be done