Android - How to uninstall "not installed" apps

Settings -> Apps -> select your app tap on menu button (3 dots) -> Uninstall for all users

If you have multiple users on your tablet, this means that the apps are installed by another user, and therefore take up space without being installed under your user account. Just open the other accounts and uninstall the apps from there.

In the menu there is an option to uninstall for all users so you don't have to loging in to each user on the main.