The most likely reason why the Decisions add-in is not showing up, is that there are policies blocking the app from showing up in the app gallery in Teams. To fix this check the following.
- Go to: https://admin.teams.microsoft.com/
- Teams apps > Permission policies
- Make sure 3rd party apps are allowed
- For each policy, review 3rd party apps and make sure Decisions is not listed as blocked
- For more information see the Microsoft documentation: https://docs.microsoft.com/en-us/microsoftteams/teams-app-permission-policies#edit-an-app-permission-policy