passcode on iphone6

Connect iPhone to Computer / Mac

Settings > Touch/Face ID & Passcode > USB Accessories > [Enable]

You haven't mentioned whether you have a PC or Mac. However...

1. Use your USB lightening cable --> Resolve USB-related alerts in iTunes - Apple Support
2. If unable to connect --> If iTunes doesn't recognize your iPhone, iPad, or iPod - Apple Support
3. If your computer is a...
   1. Mac --> If your computer doesn't recognize your iPhone, iPad, or iPod ...
   2. Windows PC —> Troubleshooting issues with iTunes for Windows - Apple Community
4. Use genuine cable --> Identify counterfeit or uncertified Lightning connector accessories ...
5. See how to --> Sync your iPhone, iPad, or iPod using iTunes on your computer ...
6. To reset the passcode for disabled iPhone -> If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...
7. Learn how to--> Update your iPhone, iPad, or iPod touch - Apple Support

If you are experiencing an issue not listed here, contact support

If you forgot your iPhone passcode - Apple Support