Javascrpit code not running in my code.

Question

     I want a make a simple ASP.NET website asking user to enter first name and last name and showing result as the full name entered when he click on the button. If user did not give any inputs for first name and last name and leaving leavings fields empty and clicking on button, the website will shown an alert please enter the required fields. For this I wrote a JavaScript code to verify whether the user entered the fields. But the problem is that whenever I clicked on button without entering any inputs it does not showing any alert message. Below I uploaded my source code, please solve this and thanks in advance