Why can’t I sign in to my iPhone 6s, an error message always pop up whenever I tried signing in

Telling us what that message actually says would probably result in an near-instant solution.

'If you can't connect or sign in to iCloud', click here --> https://support.apple.com/HT203512

"If you still need help

If you still need help signing in to iCloud, update your iPhone, iPad, and iPod touch to the latest version of iOS or iPadOS, and your Mac to the latest version of macOS. See the iCloud system requirements for more information."

For official Apple Support click here --> https://support.apple.com/choose-country-region Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world."

Click here --> https://support.apple.com/HT201232