Please uninstall your app from that store and then go to partners.shopify.com, clicks Apps, then click on your app name, then click Test your app (select your development store) and then "Install." What happens when you do that?
You also may get rejected for not implementing the Billing API. I suggest using the GraphQL instead of REST API version.
It is working fine when I text my app from partner account.
I think it may issue as below line.
Could you please check.
$install_url = "https://" . $shop . ".myshopify.com/admin/oauth/authorize?client_id=" . $api_key . "&scope=" . $scopes . "&redirect_uri=" . urlencode($redirect_uri);
This is not how you do it. You need to pick up shopOrigin dynamically (Shopify's guidance here is super thin, unless you're using one of their libraries).
This is how it has to work:
So as I stated, you need to follow the instructions in my previous message where you actually click "install" from the "test on your development store" and see what happens - this is the same way Shopify is testing your OAuth flow.