The most common reason for this is the client is logged in to an account not associated with the subscription. Confirm with the client the correct email address from your Client List.
Second most common reason is that the client is trying to purchase something that is not redeemable for their subscription. You can check what is redeemable by going to the package builder and scrolling down to Redeemable For & Advanced settings.
Another reason for this is that the client is not seeing the preselected subscription option at checkout indicated with a star. 🌟 This will always be the second option underneath the drop in price.