How to fix invalid response error after PHP app installation?

Dear Experts

Introduce issue
I have instaled app with PHP according this guide 'https://github.com/nyalex/shopify-generating-api-token-guide’.
I put these files to server. App was installed succesfull. It appeared to my development website. But when I enter to my app i get error ‘[link] sent an invalid response.’ There is showing nothing,

Could you give me code how to fix it because I am stuck ?