How can I buy domain said I need to add payment method

[Michael221]

How can I buy some items from Shopify 

[StephensWorld]

Sounds like you answered your own question ... you need to add a payment method to your Shopify store/account. 

Admin  > Settings > Billing

Once you've added a billing method, then you can purchase your domain name via Shopify:

https://help.shopify.com/en/manual/domains/add-a-domain/buying-domains