Support for Apps in Assignments

losbit 

Thanks for trying this out. Could you please check the following:

1. You are running this within an Educational Tenant. Assignments only works on Educational Tenant ( A1,A3, A5).  You can sign up for an educational demo tenant here Demos for MPN partners - Partner Center | Microsoft Docs 
2. Create a Class Team in the Educational Tenant ( this adds the Assignment App to your Team automatically. See here for how to do that Create a class team in Microsoft Team
3. For your Educational Tenant, ensure you are in the Development Ring Enable Developer Preview
4. Sideload the Tab app in the Class Team you created , in the General Channel. Assignments App support is only supported for Tab App, and not Personal Apps
5. Go to Assignments, create a new Assignments, click on Apps, and and verify that your app is listed there