No.1 Turn off Wi-Fi networking:
• Go to settings on your iPhone 6.
• Tap on Privacy.
• Tap on Location Services.
• Now Choose System Services.
• Tap on Turn off Wi-Fi networking.
• Once you have done this, turn off your device and wait for 1-2 minutes. Now Turn on your iPhone and try to connect with your Wifi network.
No.2: Reset network settings:
• Go to Settings on your iPhone.
• Tap on General.
• Tap on Reset.
• Tap on Reset Network Settings.
No. 3: Update iOS:
Open Settings on your device, Tap on General-Software Update-> Tap on install it. You may be facing this issue because you are running an old version on iOS on your device. Updating to the latest version might get rid of this problem.
No. 4: Factory Reset:
• Open Settings and Tap on General
• Tap on Reset from the list.
• Tap on Erase all Content and settings
If you have the passcode/fingerprint, enter it
• Tap on Approve ->Erase iPhone.
After trying all these solutions, still you have no luck then you should go to service center and let it checked by the staff.