Why I cannot be backup my iPhone-6 on iCloud ? Even I have 4.7GB free space and phone was plugged in charge. Please help me. Which says the last backup could not be completed

Try to reset your device. This won't delete your data stored on the phone.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

After you reset your device, try to backup it once again.