Looking to execute ansible from slack, do we have documentation?
Did you configure ChatOps (ChatOps — StackStorm 3.1.0 documentation) and installed Ansible integration pack (GitHub - StackStorm-Exchange/stackstorm-ansible: st2 content pack containing ansible integrations)?
That should be your first steps.
Besides of that, a little bit of googling might lead to the following tutorial: Ansible and ChatOps. Get started 🚀