I thought I had replied to this post earlier,but must not have submitted it. You can reach out to the Premium Care department at (866)371-9501 and explains how you're trying to add the Samsung Care+ but the option to do so is not available through the Samsung Members app. They may be able to do it over the phone for you, or at least direct you to how you can do it outside of the members app. Hope you are able to get this resolved! And just for future reference, it's easier to add it at time of purchase and cancel it later if you decide you don't want it. You can cancel the subscription online in your Samsung account.
There is a Samsung Care + banner in the Samsung members app, but when you click on it, it does not give you the option to enroll for some reason. You can call them at (866)371-9501 and see if they are able to enroll you over the phone?