This suggests that Twitch is unable to reach your callback URL, or you are not responding correctly.
This could be a SSL problem or your endpoint is not web accessable, or your response logic is wrong.
You can use the CLI to test your endpoint for conformance - GitHub - twitchdev/twitch-cli: The official Twitch CLI to make developing on Twitch easier. - twitch event verify-subscription stream.online -s "YOURSECRET" -F "YOURCALLBACK"
And you can use SSLLabs to test/verify your SSL Setup SSL Server Test (Powered by Qualys SSL Labs)