I can't log in to iCloud on my iPhone 6

Either the mail address or password that you are trying to sign in is incorrect as there might be typing mistakes in both , enter the mail address and password correctly , sometimes if incorrect credentials are entered multiple times server assumes some one is tricking and using phishy attempts .

Server doesn't know who is using the iPhone , it assumes and locks the account see the complete details in the article If your Apple Account is locked, not active, or disabled - Apple Support

The second reason if Apple ID account doesn't exist on the iCloud and it is erased from the server How to delete your Apple Account - Apple Support

And that can be verified using this link https://iforgot.apple.com/appleid#!§ion=appleid

Enter the correct credentials if the account exists a green check mark will always appear the account is verified .