Now that you've backed up your iPhone, you can erase it. Go to Settings > General > Transfer or Reset iPhone. Select "Erase ...
When an iPhone suddenly refuses to install a new app or take another photo, the culprit is often a mysterious bar in Settings ...
Your iPhone may have low storage because it's been consumed by System Data, which can potentially consume all available capacity. Here's how to regain space when things get too bloated to function.