Adult Family Member Unavailable for Sharing

Hi, I'm a apple card owner and I'd like to add another apple id as participant/co-owner, the account is already in my family sharing group, but it is shown as "Unavailable for Sharing" when I click share card.

I made sure that I'm qualified for all the requirements in Set up Apple Card Family and add a co-owner and participants - Apple Support. Both iPhones are in the latest iOS version, the region is set to US. Please help here, thanks.