|When certain numbers or letters won't type for Product key entry, it usually means those characters are not valid for that particular version of Windows.|
For example, if you are trying to perform a new installation of Windows that wasn't pre-installed at the Dell factory, the Product Key affixed to the back of the computer is not valid. That key is valid only for the pre-installed version.
So, the bottom line is that there's nothing wrong with your keyboard. You are just trying to enter an invalid key.