I am unable to add my debit/credit cards to wallet

Credit/debit cards can be added to Wallet only if you use Apple Pay.

Check if your country supports Apple Pay AND if your card issuer is among the partecipating banks in this list:--> Countries and regions that support Apple Pay - Apple Support

Should be this case, set up your Apple Pay, using these instructions:--> Set up Apple Pay - Apple Support

Regards

Giulio