Same, experienced this issue as well - Allow Network Access - "Could not grant checkout ui extension scope 'read_checkout_external_data'"
FYI though to necessarily save you time, it seems like Shopify Checkout UI Extension is only available for Shopify Pro users. If you aren’t targeting Shopify Pro ecommerce stores, you may consider alternatives such as Shopify theme app extension (customized blocks), or Shopify Remix if your app requires a full page plugin feature. Shopify theme app extension seems like a good alternative as well (right now still seeing whether it’s capable of modifying the cart items/apply discounts), and I also managed to make my React code work along with my theme app extension. (if you code in React)
You can do more research on whether Shopify Checkout UI Extension is only available to Shopify Pro users. Other people feel free to correct me if I am wrong.