Q: Photos Library is not yet available

So far so good on my own fix (above), even a sync over wifi doesn't produce the error. As far as I could tell, the sync error never prevented any other data from syncing and I could always import the new photos (those not already in photo stream) in Photos App by selecting the device from the left and importing anything new.

I had tried many methods suggested on this thread, others threads, and Apple technical note.

But, deleting the device's backup is what did it for me.

@Christian Riley.

I am exactly in the same situation. I'll try your overnight solution as all the solutions above didn't work.

This worked for me.  After the sync, I went back into Photos and re-set iCloud to cover Photo Stream.  The test will be whether the problem recurs after reconnecting to a partial iCloud (Photo Stream only) arrangement

I figured this out and fixed mine - was getting same error with Photo's (not iPhoto) when syncing with my iPhone. 

Go and find your file with your Photo Library.  I found mine by going into Finder, then clicked on Pictures folder and found file in there.  Mine was titled Photos Library 2.

Right click on it, then click on Get Info.  At bottom under "Sharing & Permissions:" I changed all 3: my user name, staff, and everyone to be Read & Write.  After this the error went away.

See iTunes cannot sync photos to the iPhone because your photo library is not yet available. Please try again later??

Downloading iTunes again solved the problem for me. I left the iPad plugged in: at the end of the download the iPad was successfully synced.

iTune 12.3.1.23 before and after the download. I suspect a cache or preference file was refreshed by the download.

Finally! Thank you! I've been trying unsuccessfully for months to get my photos to sync and have tried a gazillion things.

Rebooting the phone, rebuilding the photo library, and restarting itunes did the trick for me. Thanks again!

downloading iTunes fixes it each time for me.

i suppose i could try to figure out what files are getting changed but it's quickest to just download, reinstall and go... fixes it for multiple iPhones and iPads.