iPhone 6s is with me . Is it compatible to download latest update

Go to Settings > General > About

What does it say for iPhone Model?

You need an iPhone 6s/6s Plus or newer in order to run iOS 14

Your details say iOS 12, so I suspect you might really have an iPhone 6?

Update your iPhone, iPad, or iPod touch - Apple Support

error?

If you see an error when you update or restore your iPhone, iPad, or iPod - Apple Support

other issues?

Updating iOS: common issues and solutions - Apple Community

How can I update software latest for my I phone 6

see my post directly above yours.

If you are asking about an iPhone 6 running iOS 14, never gonna happen.

iOS 12 is the end of the line for that model.