How do I add my Walgreens card on Apple Pay?

You can’t anymore. Walgreens discontinued Balance Rewards. And their new program only lets you key in your phone number at checkout. It doesn’t have Apple Wallet integration. Leave negative feedback on their app and hope for change.

I have not done this myself but the following information from Walgreens site https://www.walgreens.com/topic/promotion/applepay.jsp may help:

Use your Balance Rewards card with Apple Pay by viewing it in the Walgreens App and tapping Add to Apple Wallet.