Receiving Error 400 on iPad when entering Gmail address

If you are preparing a previously used iPad for a new user, you need to properly erase and reset the iPad:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

After erasing the iPad, it can be set-up again "as new" for its new owner - creating a new Apple Account if the user doesn't already have one. The iPad User Guide describes the straightforward process:

Turn on and set up iPad - Apple Support

Theafs wrote:

I have an older iPad that I am giving to a family member but I receive an Error 400 message when I try to enter their Gmail address. Help please.

Which older iPad?

Which iOS version is it running?

Any iPad limited to versions prior to iOS 12 is dicy, and iOS 12 is itself getting dicy.

Various apps will further require newer versions, such as iOS 18 or iOS 26.