Hello RBA1996,
If you're not seeing an option to add a debit card to Wallet, it's possible that you're in a location that doesn't support Apple Pay. You can find out if you're in a supported location using Countries and regions that support Apple Pay.
If you're in a supported location, then you should see the options under the "How to add a debit or credit card on your iPhone" section of in Set up Apple Pay, as long as you're signed into your Apple ID on your iPhone.
"How to add a debit or credit card on your iPhone
- In the Wallet app, tap the Add button
.
- Tap Debit or Credit Card to add a new card.
- Tap Previous Cards to add a card that you used previously."

If needed, you can use the steps in Sign in with your Apple ID to sign into your Apple ID on your iPhone.
"Sign in on your iPhone, iPad, or iPod touch
- Open the Settings app.
- Tap Sign in to your [device].
- Enter your Apple ID and password.
- If prompted, enter the six-digit verification code sent to your trusted device or phone number and complete sign in."
Regards.