I've used the kyon147 Laravel app package. So what I found is that when you follow single-merchant installation link:
1. Shopify will hit authorize/ path on your app's server.
2. Your server will create a new user in 'users' table in Laravel's database and assign password.
3. Your server will send this password back to laravel, so you can install the app on the installation app page and accept app requested permissions to the store.
4. If you see the Install green button you're good and you can install the app.
5. If you will not get the green INSTALL button on the installation screen(regardless of what went wrong). Laravel database users table will have new user created and will not let you install the app again until it will be removed. Laravel is notifying shopify that installation process has beguna nd is blocking new installations.
To fix: Remove created user from Laravel database for the instal process be reinitiated.