I can't connect my iphone 6 plus to Itunes store and app store, what will I need to do?

In my case, i just go to Settings->Itunes & App Store and Sign Out and Sign In again. Then enable all Automatic Downloads (Music, Apps, Books, Updates).

PS: Enable all automatic downloads may not be necessary.

Good luck.

Although my AppleID is already exist and I connect with it on Apple website, I can't connect to iTunes & App store from the settings. What can I do?