Can't add new ACS namespace: "Not Available"

I'm trying to figure out how to set up Azure as an IdP for our web app, and most of the stuff I'm coming across is for ACS.  When I try to add an ACS namespace (it would be the first one), the link is greyed out and has "NOT AVAILABLE" in red underneath it.  Is ACS deprecated, or is there some other requirement that I'm not meeting?

Thanks!