Why am I unable to to add my cards to my Apple Watch Wallet?

Hello,

You will need to add the card using your paired iPhone.

About Wallet on Apple Watch - Apple Support

"The Wallet app on your Apple Watch lets you easily access the cards, passes, and keys you added to the Wallet app on your iPhone"

Set up Apple Pay - Apple Support

"Add a new card on your iPhone

1. In the Wallet app, tap the Add button .
2. Tap Debit or Credit Card.
3. Tap Continue.
4. Follow the steps on the screen to add a new card. If prompted, choose your bank or card issuer from the list or find them using the search bar.
5. Verify your information with your bank or card issuer. They might ask you to provide additional information or to download an app before approving your card for use with Apple Pay.
6. If you have a paired Apple Watch, you have the option to also add the card to your watch."

I hope this helps.

I'd recommend following the guide here: https://support.apple.com/en-us/HT204506