SharePoint Framework Extensions invalid application package

Question

I'm learning how to make a SharePoint Framework Extensions Apps. I made an app which is the same like in tutorial: Deploy your extension to SharePoint (Hello world part 3). Unfortunately i can't deploy apps which are SP Extensions. I even tried to upload a template app. Every time i have message:
"The requested operation is part of an experimental feature that is not supported in the current environment."

I tried with webpart apps and everything works.

Answer 1

SPFx extensions will work only on Developer Tenants.

If your tenant is on First release or General Availabilty(which looks like your tenant from the current error message) , it will not work.

To create a developer tenant, you can use either of the below 2 links:

1) https://demos.microsoft.com/Login

2) https://dev.office.com/devprogram

Reference - SPFx extensions overview