This problem seems to happen frequently. A Lightroom user thinks they are up to date when in fact they are still using the initial installation of Lightroom CC 2015. The Creative Cloud application manager indicates they are up to date. We provide the link to download the latest update of Lightroom, and when they try to install the update fails. I don't know how to help them. I refer you to this link:
Lightroom CC ARC won't update
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