Update (September 1):
Google is fixing Photos/Drive video sync issue case–by-case. So, you’ll have to contact them individually, as they have not disclosed any general fix for the issue yet.
Original Story below
Facing sync error with Google Photos videos on Drive? Looks like you aren’t alone. For quite long, users have reported failure to sync video files from Google Photos to Google Drive.
The issue dates back to October, 2016 when it was reported for the first time. Both Windows and Mac users note that Google Drive software fails to sync video files uploaded in Google Photos. A message saying “can’t sync # file. An unknown issue has occurred” pops up, irrespective of the file size. Most of the users have reported problem with .mp4 and .mov format.
On January 12, 2017, a Google employee confirmed that they were aware of the issue by saying:
“No updates to share yet at this time, but we’re looking into it (CM #34223459)”.
Furthermore, she added:
“An alternate workaround in the meantime would be to download the content from the web, re-upload it, and finally delete the bad originals”.
A quick look into the relevant thread on Google Drive official forum reveals that this issue continues to bother users till date. If you are facing the same problem, rest assured that we’ll give you a heads up as soon as an update is made available.
A couple of users have provided some workarounds that fixed the issue for them. Take a look at the screenshots to get a clear idea.