I've created a Teams bot using the 'Post messages to Teams' template from VS Code Teams Toolkit plug v4.1.3.
After provisioning and deploying it to Azure, sending a POST request to it results in a 400 Bad Request with the following response:
"Error: validateAndFixActivity(): missing activity type."
It works fine if I run it locally.
There are several APIs for a 'Post messages to Teams' app, and please make sure you are sending POST to '/api/notification', e.g.,
POST to https://{your-azure-host}.azurewebsites.net/api/notification
In addition, the 400 Bad Request may be returned by another API /api/messages which is used for Bot Framework Connection.