What step are you on in the process?
If you are still setting up your webhook, please make you are using the following format for your webhook URL
https://www.your_domain.com/webhook/twitter
If you have already received a webhook ID and are working on the CRC check, please make sure you are adding that webhook ID to the end of your endpoint:
https://api.twitter.com/1.1/account_activity/all/:env_name/webhooks/:webhook_id.json
Let me know if either of those recommendations helps move you along.