Make sure your Teamwork account is at the Pro level or higher.
If you upgraded to a paid Teamwork plan after connecting your Teamwork account to Zapier, please use the Disconnect button next to your Teamwork connection at this link: https://zapier.com/app/settings/authorizations. Then, click to connect that Teamwork account from scratch. That will ensure the connection includes the details about your upgraded plan.
Make sure you have webhooks enabled on your Teamwork account, as in the screenshot below.