You need to quit the app before uninstalling it, if that don’t work, you can always force quit the app. There are several ways to uninstall an app but we recommend you to use one of the below options:
- The easiest is to open Launchpad, and then drag and drop the icon to the Trash, or press Alt and then click on the little X
- You can also open the applications folder in a finder (use Go menu / Applications), and then delete or move the app to the Trash.
You can reinstall an app again from the App Store, Apple won’t charge you again.