find my iphone

Why not? What happens when you try? How are you trying to turn it off exactly?

See here if you cannot access the iPhone directly due to damage: Remove a device from Find My iPhone on iCloud.com - Apple Tugi

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.      

You can also try iCloud.com/Find My Phone using a computer browser to remove it from your account.

Find My Phone – Remove device.    

Are the reads below related to problem you are seeing? I can't guarantee a fix, but if pertinent see:

Verification failed : There was an error connecting to apple id server https://discussions.apple.com/thread/7238348

Error Connecting Apple ID, Verification Failed. How-To Fix https://appletoolbox.com/error-connecting-apple-id-verification-failed-how-to-fix/

Seems like a problem with the connection. Is the iPhone connected to the internet in some way?