Error message when adding a recovery contact

I’m setting up a recovery contact in my son’s phone. I already have a “family” set up. I’m on the screen that says “add recovery contact” and I am clicking on my name (I’m also the family organizer). Then I hit “Next” and I get this error message: “Cannot Add Mom. You can only add contacts who are using iMessage.”

I have an iPhone, it’s up to date, and I am using iMessage (and yes, I double checked that it was turned on in Messages). I then tried the same steps with both my husband and sister, both of whom are in our “family” and have up to date iPhones and use iMessage.

Any idea what’s going on and how to fix?

The whole reason I’m in this mess is bc I didn’t know I had this option and my son forgot his passcode and I had to reset his phone!

TIA