Why is View on the frontend link not working?

In the Form admin, if you want to see how the form looks in the frontend, you will use the View on Frontend function which will output the form's URL in the following format

https://[store_name].myshopify.com/apps/product-registration?form=[form_slug]

If you changed the link e.g /apps/product-registration bit to something else, then the View on Frontend link won't work, you will need to replace it with the app proxy you changed to

The app proxy is shown in the Shopify admin > Apps > My Product Registration > ... (top right) > App Proxy

If the app proxy is changed from the default, you will need to use the new format for the URL

https://[store_name].myshopify.com/[app-proxy]?form=[form_slug]

If your store uses a custom domain, the URL format will be

https://[your-store-domain]/[app-proxy]?form=[form_slug]

or

https://[[your-store-domain]/apps/product-registration?form=[form_slug]

It's not recommended to change the default app proxy, because some of the links will not work out of the box, so weigh the pros and cons before changing the app proxy