To add more templates here you need to create template in live theme. If you are trying with duplicate theme then it will never work.
And if you want to know, how to create alternate templates you can read here https://www.shopify.com/in/partners/blog/shopify-alternate-templates
Hi, @theminepetshop . I’m happy to help you with this! What kind of layout changes are you making to your product pages?
As @Guleria mentioned, the only templates that will appear as options in that area are templates that exist within your current theme, the one currently live on your online store. You can double check which theme you’re using as your current theme by going to Settings > Online store > Themes in your Shopify admin.