Trying to create Hello world using SharePoint Framework Extensions but no "allow manifest" message

Hi,

I am trying to create Hello World message using  SharePoint Framework Extensions but no "allow  manifest" message , nothing happen.

Using this example 

https://dev.office.com/sharepoint/docs/spfx/extensions/get-started/build-a-hello-world-extension

Tried on mutiple office tenants and sites same issue