Permission Denied for a small set of photos when adding to macOS Mojave Photos
I'm not sure what caused the problem. I went to iCloud.com, opened Photos and was able to upload several of the bad photos without an issue. 

I shut down macOS Photos and re-launched it after it had completed syncing the batch of files it was syncing to iCloud. Reopening Photos had the bad photos I tested on iCloud.com sync down without issue. I then imported the rest of the bad photos and they were added successfully. 

I assume something was messed up in Photos and a restart was needed.