In case the store had another registry app previously installed, the merchant may have to update the app proxy setting in the Shopify Admin Panel. If you are getting a 404 error when trying to access a page address like yourstore.com/apps/giftregistry/find then this article likely applies to you
To update the App Proxy -
Step 1: Go to Shopify Settings-> Apps and Sales Channel section and click on Gift Reggie app
Step 2: Click on "Customize Url" under the App proxy section
Step 3: From the pop-up, ensure that the input box to the right has giftregistry not giftregistry-1 or any other value, and save!