Having issues installing plugins

The last 4 items in your app.yml file that you are trying to add as plugins are actually theme components. You can see that if you go to the URL for each of those items on Github and look at the repo’s README file. For example: https://github.com/discourse/discourse-header-submenus.

Installing a theme component is easier than installing a plugin. Have a look at this topic for details about how to do it: How do I install a Theme or Theme Component?