Unable to add a recovery contact - it says the contact must be using iMessage (which, they are!)

I'm trying to add my husband as a recovery contact. We both have iPhones, Macs, and one iPad - both iPhones are set up for iMessage.

Every time I add him as a recovery contact, I get an error indicating it must be someone who is using iMessage.

iMessage is turned on on both of our phones. Also tried adding my son, same problem.

I see a list of send/receive addresses in the iMessage section of settings, his info is not there, nor is my son's. I don't see any way to add them or add any additional send/receive addresses

How can I get past this problem?