This usually happens when the Internet connection was lost while downloading the app.
The usual fix for this is to restart your device.
1. Press and hold the side button until you see the red power off slider. Drag the slider right to turn off the device completely.
3. Press and hold the side button and wait till the white Apple logo appears.
After that, the app should no longer be listed as installed in iTunes and you'll be able to download it again (don't worry, you won't be charged twice).
Hope it helped and if you have other questions, write us at firstname.lastname@example.org.