trying to clear old iphone before selling

If you're unable to connect to the Apple server, you've got a network issue. Go back in settings to Wifi. If it shows you're connected to your network, disconnect and then reconnect. Then you should be able to sign out and then erase your device. If you still have trouble connecting to the Apple server, sometimes it helps to disconnect from wifi, restart your device, then try to sign out again. You also can restart your modem to see if you get a better network connection. If all else fails, with your device powered on and connected to wifi, log in at icloud.com/find>click all devices>click on this device>click erase>click remove from account OR on the Find my iPhone app on an iOS device>click this device>click erase>click remove from account.

https://support.apple.com/en-us/HT201274

https://support.apple.com/kb/ph2702?locale=en_US

Apple's suggestions for clearing the phone.

Selling/Giving Away Device

Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch

Try a forced restart.

Reset Device

If necessary

Hard reset

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting

Backup - Restore your device from an iCloud or iTunes backup

Did you try erasing from iCloud.com/find or through the Find my iPhone app on an iOS device? As long as your device shows as online, you'll be able to erase and remove it from your account.

To speak with an advisor via chat, go to getsupport.apple.com.