Your control is server side controls and your validation code is in Javascript, so you cannot access to the control like when you're coding at server side.
In your Javascript, try this to access the textbox:
if (document.getElementById("TextBox2").value.length == 15)
Use this with your radio buttons:
if (document.getElementById("RadioButtonList1")[0].checked)
and
if (document.getElementById("RadioButtonList1")[1].checked)