That is an error that is associated with your database. You are probably inserting the results of the form into a database, the column "Player_First_Name" you have marked as not allowing null entries... it only happens when you leave that field blank.
The underlying problem is that no validation is taking place at all... looking at your page it doesn't appear you have any form validation applied. I looked and the form doesn't have an onsubmit event where the code would usually be added.