Integrating Teams chat in a model driven app, doesn't work.

In our company we have also Dynamics 365 Sales and there it works.

It doesn't seem to be a licence issue.

We followed all the Microsoft instructions on:

https://learn.microsoft.com/en-us/dynamics365/sales/teams-integration/enable-teams-chat

So, the question is, where is the problem?

How can we enable and use Teams in our model driven app?

The error we receive is: “No manifest found”.