iCloud photo sharing not working
I used to be able to see shared albums on my MacBook Air. (High Sierra 10.13.3) They are gone and when I try to subscribe to a new shared album I get a message saying that iCloud photo sharing is not available for this device. Tells me I must be signed in to iCloud on my phone and computer. I am, and I have activated iCloud photo sharing on the MacBook. Don't understand why I can no longer see shared albums. Thanks for any advice.
MacBook Air, iOS 10.3.3