iPhone error 78 is among various iPhone or iTunes errors. To fix the issue on your iPhone 6, you could try the following ways.
1. Hard reboot your iPhone. (data loss risk) Quickly press and release the Volume Up and Down buttons respectively, then hold the Power button.
2. Restore iPhone via iTunes. (data loss) Connect iPhone to compute and open iTunes, go to Summary> Restore iPhone.
3. Use professional iOS system repair tools such as
TunesKit iOS System Repair. (no data loss)
Step 1: Launch the software and connect your iPhone to PC.
Step 2: Choose the Standrad Mode to fix without data loss.
Step 3: fix the iPhone error 78.