You have to check your storage device and delete some of data such as photo, video, app and other data. If you delete all unnecessary data but storage of your iPhone, iPod touch and iPad also full. Maybe data that make your device storage full are cached files.
This is one of the choice to clear you iPhone, iPod touch and iPad cache. Make more space from junk file, old app data or cache data. First you have to check your total storage available on your device by go to Setting > General > Storage & iCloud Usage. Then go to Appstore and find out apps, game or movie that have file size bigger than available storage.
Tap the app and tap the price on the right side of the screen, then tap Buy. To get a free app, tap Get on the right side of the screen, then tap Install. Click Cancel when have password require for your apple ID or that will have Cannot Download windows show up then press OK. Check your total storage available on your iPhone, iPod touch and iPad so you will have more storage available.