I have done two posts on automating posting messages to teams.
In the course of posting a message via the HTTP end point, you should get a response as follows:
However there are times you may get this response
The resource you are looking for is unavailable
If you get this error, the problem is your end point.
Either you have a typo in the URL or you have not updated the channel URL.
How you check for the latter is by looking at the URL
If it starts with
Then it is out of date and you need to update it. Follow the instructions here to access the existing webhook. If it is out of date, there will be a button to update it.