Unable to update my Asus ZenFone 5

I found my old ZenFone 5 and want to give it to my little brother. But I can't seem to get it to update beyond 4.4.2 . I don't exactly remember what I did with it back in the day when I had it. I think I tried unlocking the bootloader or something.

When I try to apply the update from within the phone, I get this error on the screen



Please help!

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