I am currently facing issues while trying to preview my PHP app in my Shopify store and partner account, where I have the theme app extension installed. When I run the command "npm run dev" in my command-line interface (CLI), it displays a "505 SERVER Error." I would appreciate guidance on how to resolve this issue.
I am new to Shopify app development and have a specific goal: creating a Shopify app in php with theme app extension in to handle "Back in Stock" notifications. The app should provide notifications to customers when a product or its variant is unavailable or sold out. To achieve this, I plan to add a dynamic "Notify Me" button below the payment button on the product pages of Shopify themes. This button should only appear when the product or variant is unavailable and disappear when it becomes available again.