That's not good 😞
If you choose a free theme does that download as it should ?
If it does and your info in Samsung Billing is right and Samsung Billing is up to date it could be your Bank rejecting the transaction.
If your unable to try another card then perhaps call your Bank and ask them to check things at their end.
Current Phone ~
Samsung Note 20 Ultra 5G SM-9820 _256Gb SnapDragon 865+
Samsung One Ui 2.5/ Android 10.