Some TLD's require that the domain is actually added to the new nameservers before being able to change them at the registrar.
You should be able to add the domain in your account without changing nameservers though (wasn't possible with cPanel but after the change it is an option), once you done that you can try changing them again.
If you still have problems, try to get a picture that shows a bit more of the page (though from the little I see you are at the right place and the values are correct).