App developers usually issue updates for their apps shortly before or after the introduction of a new iOS version. This is their strategy for keeping their apps compatible with the new phone platform. Due to recent changes in the system requirements, the app is likely to stop operating correctly if these updates are not installed. You'll need to manually check for and install pending app updates on your iPhone 7 Plus unless you've configured Spotify or other applications to auto-update. Here's how you do it:

1. From the app store
2. Tap the Updates symbol in the lower right corner of the App Store main screen. The screen then displays a list of applications that have outstanding updates.
3. From the list, select Spotify and then hit the Update icon next to it. This will install the Spotify app's pending update.
4. If you notice many app updates, you may update all of them at the same time. Simply press the Update All button in the top right corner of the screen. 
5. Allow your applications to finish upgrading, and then restart your iPhone to implement the new modifications from your just installed app updates. Then open Spotify and check to see whether it's already up and running.