I had this issue with my MacBook Pro (Intel). I took it to the Apple Store and they somehow managed to install the update. Then the issue popped up on my iMac (Intel). After trying all the steps outlined in this and other forums (recovery mode, etc), I finally figured out that I needed to boot off of a separate drive to install the update. I created a bootable USB drive using the instructions here: https://support.apple.com/en-us/HT201372.
Once I booted from that drive, the install went flawlessly.
Good luck.
aviduser
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