macOS Sierra / Numbers 4.0: Problem with saving on a Synology via AFP
I found this thread while looking for a resolution to a problem with my ReadyNAS device.  The root cause appears to be that Apple removed the AFP protocol from macOS Sierra, at least according to comments on Apple's discussion boards.  SMB still works, but AFP has either been removed, or at least deprecated.

https://discussions.apple.com/message/30684682