I am unable to purchase USDT through Apple Pay

You cannot put a Wallet in a Wallet. Apple Pay requires the use of Apple Wallet. When cards are added to Apple Wallet, you can activate Apple Pay with a double click of the iPhone side button and pick the card of your choice that you’ve added to Apple Wallet.

You can have your payment cards in more than one wallet app, if the issuing bank supports adding their cards to multiple wallets.

Set up Apple Pay - Apple Support

Make purchases using Apple Pay - Apple Support

Can you please explain in more detail what you’re trying to do? what country are you in?