I deleted once again all subscriptions, then run createSubscription (it gave me response code 202). I got challenge, timeouted for 2 sec and gave challenge back via
function doGet(e){
Utilities.sleep(2000);
var ss = SpreadsheetApp.openById(XXXX).getSheetByName(XXXX)
var chall = ss.getRange('A4').getValue();
return ContentService.createTextOutput(String(chall));
}
It was pending for a while. After few minutes subscription status was webhook_callback_verification_failed.
P.s. I will make a guide how to use GAS with Twitch when I’ll be finally done with that. Poor new enthusiasts like me…