If checking out with Shop Pay, when you first go to checkout and are directed to Shop Pay, go to “checkout as guest” within Shop Pay. This will direct you back to the checkout where you can select to use rewards points/discounts/gift cards. Once you submit that, you should be able to get back into Shop Pay to complete the order.
(Alternate method): If you usually redeem your rewards points prior to checkout, you can also add your reward code to the “Add a discount code or gift card” section in Shop Pay. First, you will want to go to the Rewards page and click Redeem My Points. Make sure to copy the reward code. Then you will paste that reward code in the “Add a discount code or gift card” box in Shop Pay.