Cannot import widget.tar.gz

[buuuudzik]

I've tried import into Mosaic newly created widget, my first one, I wanna try how it works but unfortunately probably it doesn't work.

POST /apps/data/mosaic30/api/?a=cwidget&name=Elevator&key=buuuudzikelevator

ends with status 404 in Network tab and widget doesn't appear on Extension list.