I cannot update my HTC 826 to android 6.0

[AC Question]

Every time I try to update the android version to 6.0, it aborts with following error:

E:missing bitmap oem_unlock_bg
(Code -1)
E:missing bitmap oem_unlock_bg_yes
(Code -1)
E:Find no match PARTITION: for package path @/cache/recover/block.map
/dev/block/bootdevice/by-name/system has been remounted R/W
reflash device to reenable OTA updates
E: Error in @/cache/recovery/block.map
(Status 7)

I have never rooted my phone.

Please help.
Thanks in advance