When you try to use an app on Android you get this error:
App isn’t installed
You get the App isn’t installed error when:
The app is currently being updated
The app is installed to the SD card, rather than running from main memory (and the SD card is having issues or the app doesn’t work well from the SD card
The app is corrupted
To fix the App isn’t installed error, check that it’s not currently being updated, failing this, reinstall the app to the main memory (either move it, or preferably uninstall it then make sure the app isn’t installed to the SD card).