Why does the poison not work? What to fix?

Question

PHP

Good afternoon, I inherited the project, I don’t have much experience with the integration of payment systems. In general, the problem is this, I don’t understand and don’t know where the error is
https://pastebin.com/e4TNajhv here is the payment page code, I’m trying to test the payment, I enter the amount, I go to Yandex.money and then writes “something went wrong ", the reasons for the failure of the transaction are not written, maybe someone knows what could be the matter?