SharePoint Framework - App File (.sppkg) Upload not working

Issue - On upload of app (.sppkg) file, App package not getting valid. On uploading, it is expected to SharePoint will display a dialog and ask you to trust the client-side solution to deploy.

However, popup is not coming. Also, Product ID is also blank

Reason - Your Tenant is not a Developer Tenant. SharePoint Framework is not available in production environment, so the only way you have to test it, is using a dev tenant. 

Resolution -

1. Follow the steps to create a new Office 365 Developer Tenant - https://dev.office.com/sharepoint/docs/spfx/set-up-your-developer-tenant

2. Convert existing tenant to Developer Tenant -  Make sure you have First Release options enabled.

After updating First Release settings, it may take a while to update the settings. Once updated, your tenancy would have SharePoint Framework Release enabled and App Packages will function as required.