this means that Shopware is not able to find your template files. Shopware does not know that you have custom template files in your plugin. For that you need to call a method named addTemplateDir(). This is possible in different ways, so depending on the plugin system you use und where in your plugin the template directory has to be added, the calling is slightly different.
For more help, it would be nice if you could provide your plugin structure
That looks quite good.
But I don’t think that you need to call the extendsTemplate method here. This method is only needed if you want to extend existing backend modules of Shopware. Your own module should work without the calling of the extendsTemplate method