I have been trying to add my card but unable to do that

See the following Apple articles:

Set up Apple Pay

Set up Apple Pay - Apple Support

Set up Apple Pay in Wallet on iPhone

Set up Apple Pay in Wallet on iPhone – Apple Support (UK)

See also ⤵️

Countries and regions that support Apple Pay

Countries and regions that support Apple Pay - Apple Support (EG)

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

In some countries is only possible to pay with Apple ID balance, such as

India

• Apple ID balance (from adding funds)
• Net Banking4 (adding funds only)
• UPI4 (adding funds only)

Check -> See -> About billing for Apple subscriptions and media products in India

About billing for Apple subscriptions and media products in India - Apple Support

Then see the following Apple articles: 

Add money to your Apple ID balance

Add money to your Apple ID balance - Apple Support (IN)

What you can buy with your Apple Gift Card or Apple ID balance

What you can buy with your Apple Gift Card or Apple ID balance – Apple Support (UK)