Insufficient Storage Error(iPhone 6s Plus)

Try the steps outlined here: How to check the storage on your iPhone, iPad, and iPod touch

Use iTunes to check the storage on your iOS device

1. Open iTunes on your computer.
2. Connect your iOS device to your computer.
3. Select your iOS device in iTunes. You'll see a bar that shows how much storage your iOS content uses, divided by content type.
4. Move your mouse over a content type, like Other or Apps, to find more information:
   

Here's a list of the types of content on your iOS device, and what each type includes:

• Audio: Songs, audio podcasts, audiobooks, voice memos, and ringtones.
• Video: Movies, music videos, and TV shows.
• Photos: Content in your Photo Library, Camera Roll, and Photo Stream.
• Apps: Installed apps. The content of the apps is listed under Documents & Data.
• Books: iBooks books, audio books, and PDF files.
• Documents & Data: Safari Offline Reading List, files stored within installed apps, and app content like contacts, calendars, messages, and emails (and their attachments).
• Other: Settings, Siri voices, system data, and cached files.

About cached files in "Other"

iTunes categorizes cached music, videos, and photos as Other instead of actual songs, videos, or photos. Cached files are created when you stream or view content like music, videos, and photos. When you stream music or video, that content is stored as cached files on your iOS device so you can quickly access it again.

Your iOS device automatically removes cached files and temporary files when your device needs more space.

If storage on an iOS device differs from what you see in iTunes

Since iTunes categorizes cached files as Other, reported usage for Music or Videos might differ. To view usage on your iOS device, go to Settings > General > [Device] Storage.