A little info, I have a T-Mobile Samsung Galaxy S21 FE. All my cards are Visa.
Since Samsung Wallet switched from Samsung Pay I have not been able to activate my cards.
All my old cards were carried over but activation attempts give me a "failed to connect to server".
I have tried force stops, clearing the cache, re-installing, rebooting, updating my phone software, removing then adding the cards back, using the internet with and without WiFi.
I messaged support and they basically told me everything I already tried except forced stops. At this point I rather use GPay.
I didn't have an issue with my visa card. I had to my bank and approve it. Maybe cause I have one card. Try adding them one at a time once a day