Always start by reading the documentation:
Document: getElementById() method - Web APIs | MDN[
^] - it's pretty clear that it returns a
null
when the element with that ID doesn't exist:
Quote:
An Element object describing the DOM element object matching the specified ID, or null if no matching element was found in the document.
So now you need to know what - exactly -
txtDDAExpiryDate.ClientID
is returning from the server, then try to match that manually against your HTML.
That'll tell you where the problem is: Server side or Client side.
And when you know that, you should be able to solve it.
Use the debugger (or just write it on the page) to find out.
Sorry, but we can't do that for you - time for you to learn a new (and very, very useful) skill: debugging!