Most likely, the gift was marked as purchased on your reservation page, which removes the ability to buy it. To update this, return to your reservation page with Babylist. You can get there by clicking on the link that we sent you over email.
If you can't find an email from us about your reservation, be sure to check your spam folder. If you have access to the registry you were shopping off of, you can also click on the item you had originally reserved directly from the registry to have the link to your reservation page sent to you via email.
From there, uncheck "I already purchased this". This will bring up the red button(s) for each item. Click those to be re-directed to the retailer where you can purchase your gift.