Cannot log out from iCloud to turn off Find My iPhone because the account is locked I can't log out from iCloud on an iPhone 6 Plus. After I enter the password, a popup is shown saying I need to unlock the account because it's locked for security reasons.

I can use the same Apple ID on a different iPhone normally.

How can I turn off Find My iPhone?
It looks like your account has been locked for security reasons, most likely because of incorrect password attempts and changes.

According to this Apple support article, you can either unlock your account at iforgot.apple.com or wait one day to get a few more attempts.

If neither of these options work for you, you can also contact Apple support and they will help you out.

Keep in mind that you will likely have to prove your identity if you contact Apple or try to unlock your account yourself, so be ready for that.