can't uninstall a harmful app
You have a few options:


instead of uninstalling the app through the notification, uninstall the app directly from the "Apps" menu in your settings. Click "Settings", scroll down to "Apps", find the app in question and tap on it, depending on the specific version of Android you have the options will vary, but you should have an "uninstall" button. Press that.
some apps are a bit nasty to remove and require root privilege. If the above doesn't work, consider rooting your phone so you can get at the app and uninstall it. This should only be required if the app came pre-installed on your phone, or if the app is truly being nasty and blocking the uninstall.