Microsoft Teams returning "400 - Summary or Text is required."

GitLab Community Edition 11.4.5

I followed the instructions to configure an integration to Microsoft teams here (Gitlab) and here (Microsoft), however, the only response so far has been:

and when viewing details, at the bottom it states:

Response body:
Summary or Text is required.

There are so few options, I am uncertain what, if anything, I can change to fix this.

Help!