Hi,
I guess this extension project wasn't created in Web IDE?
This is a problem.
One thing that comes to my mind is its project.json file probably missing the "extensibility" block with all the relevant information.
So I would recommend to create the extension project in Web IDE and add your extensions and modifications to it.
If you don't want to do that you can create it and just copy the "extensibility" block from its project.json file to your file. But there might by other issues too.
Regards,
Michal.