You can also use the TwitchCLI to test your Callback
I did so and the result is
twitch event verify-subscription cheer -F https://tagged-play.ru/twitch-callback
✗ Invalid response. Received null as body, expected 0f4f2e05-42b1-e6a7-4e2e-ebbf7e7c8aad
✗ Invalid content-type header. Received type application/json with charset UTF-8. Expecting text/plain.
✔ Valid status code. Received status 200
Which as you can see is incorrect as you don’t echo back the challenge as dist notes