Q: I can't log out icloud on my iphone 6plus

Howdy Cuongxuka,

It sounds like your other devices have no trouble signing out of iCloud but the iPhone 6plus is having an issue with that. I would quit the Settings app on your phone:

Force an app to close in iOS

1. Press the Home button two times quickly. You'll see small previews of your recently used apps.
2. Swipe left to find the app you want to close.
3. Swipe up on the app's preview to close it.

 
When you have done that restart the device and test the issue again:

iOS: Turning off and on (restarting) and resetting
 
If the issue persists I would next reset the network settings on your phone and try again:

Tap Settings > General > Reset > Reset Network Settings. This also resets Wi-Fi networks and passwords, cellular settings, and VPN and APN settings that you’ve used before.

From: If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

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