iCloud Sign-in Problem

I'm not able to sign in to iCloud on my iPhone6.

When I try to setup icloud on this device, after entering apple id and passowrd, I see this message:

"Software Update Required

Update this iPhone to the latest version of iOS to sign in."

This is while my iOS version is 12.5.7 (up to date) and there is no new update available for it.

I really need my notes, contacts, file and everything on my icloud on this device as well.

My question is that how can I setup icloud on my old iphone (6)?

Will it be helpful if I disable two-step security on my icloud account?