Depending on your device and version of Android™ installed, the internal storage  can be managed differently.

It can happen that the download of an app is refused even if you still have some storage capacity indicated in   (or) Settings >  Storage.

The storage memory on your device can be split in two parts: one for system and apps and one for third-party files (music, photos etc...). You will find below some advice to better manage your storage :

• Delete all installed apps that you no longer use. Some, especially games, are very large. If you never launch them, it's better to uninstall them. 
• Move your data on the micro-SD card in your device. The result of this procedure depends on the developer and the application itself. 
• If you have a unified storage (no separation between the party for system and apps and the one for files storage), move as much as possible all your multimedia files to the micro-SD card.