When I try to invite my spouse as a co-owner I get "Apple Card Unavailable in Your Region." We are in the US. How can I add my spouse?

Please verify the Region setting on you iPhone using information in this support article,

Change your Apple ID country or region - Apple Support

If it is set correctly to US, please toggle the region to another, then toggle back to US and lastly restart your iPhone.

If you’re behind a VPN please turn it off.