This is simply a store error, try after a few hours, but if you can't wait do the following:
- Go to windowsphone.com via laptop or PC
- Search for your app
- Click on install and wait...
If that doesn't work;
- Go back to the app page
- Scroll down and click on download and install manually
- Save the file to your desktop
- Connect the phone to your laptop/PC
- Drag and drop the file into the phone
- Go to the store app on the phone
- Tap the ellipsis button (Three dots)
- Select install "local apps"