Twilio developer evangelist here.

That's weird, because that worked for me when I was answering your previous question.

What you can do instead is return a string that determines whether the caller is blocked and the use a split widget to choose.

So your code could look like this:

exports.handler = function(context, event, callback) {
  // List all blocked phone numbers in quotes and E.164 formatting, separated by a comma
  let numberBlock =["+17171234567"];
  let blocked = true;
  if (numberBlock.length > 0) {
    if (numberBlock.indexOf(event.From) === -1) {
      blocked = false;
    }
  }
  if (blocked) {
    callback(null, "blocked");
  }
  else {
    callback(null, "accepted");
  }
};

Then the studio flow would look like this:

That is, if the function returns "accepted" then the call moves on, otherwise that's the end of the flow.