Javascript : not working even with <%=textbox1.Clientid">, when included in masterpage

Hi all,
Hope to get few answers thsi time atlease. I have been posting questions, but failed to get even a comment for this : https://www.experts-exchange.com/questions/23914095/Tree-Menu-navigation-based-on-User-Session-Can-sombody-have-a-look-atleast.html

But the real question for this post is here: I have included my content page with the masterpage.
i have a textbox.The user is not supposed to enter more than 3 chars. I have set the maxlength of the textbox=3.
But i also want to alert the user " cannot enter more than 3 chars". I call a javascript function. It was all working fine as an individual .Aspx page.
now i have included that in a master page. Now the above said error come, when i try to enter  anything in that textbox.Here is piece of code.
Plase help..Hope to get answers SOON

'This is the javascript code
<asp:Content ID="ContProduct" ContentPlaceHolderID="MainContent" Runat="Server">
   
   <script type="text/javascript" >
 
    function ValidateTxtarea()
 
    {
 
    if (document.getElementid('<%=txtActivityCode.ClientID%>').value.length >= 3) 
        {
        alert('Activity Code Cannot be more than 3 Chars')
 
        return false;
        }
        else
        {
        return true;
 
        }
 
    }
 
    </script>
 
'this is how the textbox is :
 <asp:TextBox ID="txtActivityCode" runat="server" Onkeypress= "return ValidateTxtarea()" MaxLength="3"></asp:TextBox>

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