How to set up a payment from the application in the iframe of classmates?

Question

Classmates

In the application settings, I registered a callback link to my server for processing the payment.
In the OK iframe, I launch the application, try to buy game currency, a dialogue window opens prntscr.com/fdtufk After clicking on the ORDER button, I get the following response prntscr.com/fdtvca The
request is not received on my server. What could be the problem? Could this be related to getting the VALUABLE_ACCESS right?