Try this code
$(document).ready(function(){
$('.sendButton').attr('disabled',true);
$('#message').keyup(function(){
if($(this).val().length !=0){
$('.sendButton').attr('disabled', false);
}
else
{
$('.sendButton').attr('disabled', true);
}
})
});
Check demo Fiddle
You are missing the else part of the if statement (to disable the button again if textbox is empty) and parentheses ()
after val
function in if($(this).val.length !=0){