So you are all set and got the free app called NAPA but it's keep on showing you with the message that you can not add this app here.So what's going wroing????
So here is what you should do.
Just add the “Napa” Office 365 Development Tools app to your SharePoint Online Developer Site, launch it, and you will be ready to create your first app for Office or SharePoint.
To enable Napa for App Development, please follow below steps
• In your Office 365 Admin Panel, choose SharePoint
• Create a Private Site Collection with Developer Site template.
• Browse to the newly create Site Collection and then select on first tile “Build an App”
• Now Click on ADD IT and Trust it and you are good to go.