Salutations gracesun6181,
It sounds like you have an Apple Watch that is set up with Family Setup. If that’s the case, though Apple Cash is available to users under the age of 18 and in the US, Apple Pay using a credit or debit card in the Wallet app is not available with Family Setup.
If instead you mean that you paired an Apple Watch with your iPhone and you are the user of both devices, but on your iPhone are unable to add a payment method in the Wallet app, please use the steps from the following article which goes over what steps to take if you can't add a card to the Wallet app to use with Apple Pay. If you can't add a card to Apple Wallet to use with Apple Pay
1. Check that you're able to use Apple Pay:
• Contact your card issuer to see if they support Apple Pay.
• Update to the latest version of iOS, watchOS, or macOS.
• Confirm that you have Face ID, Touch ID, or a passcode set on your device.
• Make sure that you are in a supported country or region.
• Check that your device is compatible with Apple Pay.
2. Check for service outages or connection issues.
3. Contact your bank or card issuer if you see the following error messages while adding your card to Apple Wallet:
• Could Not Add Card
• Invalid Card
• Card Device Limit
Apple does not approve or decline cards for use with Apple Pay. If your card was declined, contact your bank or card issuer.
Thanks.