Q: How do i restore GarageBand on iPhone 6s

Try to  install from your Purchases history - see this link:  Download your past purchases - Apple Support

Redownload an app

On an iPhone, iPad, or iPod touch

1. Tap App Store.
2. On an iPhone or iPod touch: Tap Updates, then tap Purchased.
   On an iPad: Tap Purchased.
3. Scroll to find the app that you want to download.
4. Tap the Download icon . After the app downloads, you can open it from your Home screen.

Did you change your AppleID or change the country of your AppleID?

Use the old AppleID, valid in the country of the store where you purchased  the iPhone.   If that is not possible, contact the App Store Support for help.