How can I add my Apple Card to my MacBook Air

You don’t need to add it because in your iPhone settings wallet & Apple Pay you can see on the bottom Allow payments on Mac , when you are in your Mac and in a website where you are going to purchase something receive Apple pay Payment then it will ask you to confirm the purchase using your iPhone Apple Pay where is stored your Apple Card

How to add Apple Card to your Apple devices - Apple Support

Ozzie1910 wrote:

You don’t need to add it because in your iPhone settings wallet & Apple Pay you can see on the bottom Allow payments on Mac , when you are in your Mac and in a website where you are going to purchase something receive Apple pay Payment then it will ask you to confirm the purchase using your iPhone Apple Pay where is stored your Apple Card

https://discussions.apple.com/content/attachment/cb913579-26bf-4a32-bcd4-fa84e313559c

That is for Apple Pay. @dialabrain linked to the correct answer to add it to the Wallet on Macs.