How can I hide an HTML table row <tr> in aspx file and enable in code behind?

An id by itself is just a client-side identifier. In order for this to be referenced as a server-side object it needs to be a server-side control. The easiest way would just be to add runat="server" on the existing element:

<tr runat="server" id="srchResultHeader" style="display: none;" class="header" >

In this case you probably don't even need the style attribute, since you're controlling the hide/show functionality in server-side code. You can just set .Visible on the control to determine whether or not it renders to the client-side markup at all.

.aspx

  <tr id="divDriverName1"  runat="server" >
<td >  
<label class=" ">label1 </label>
<asp:TextBox ID="TextBox1" runat="server" class=" form-control"></asp:TextBox>  
</td>  
</tr>

.aspx.cs

           ContentPlaceHolder myPlaceHolder = (ContentPlaceHolder)Master.FindControl("ContentPlaceHolder1"); 
         HtmlTableRow ct = (myPlaceHolder.FindControl("divDriverName1")) as HtmlTableRow;
             divDriverName1.Attributes.Add("style", "display:none");

You could use server-side <asp:Table> for this very purpose. Otherwise <tr> is a client-side thing and is not directly accessible in the server-side code. <asp:Table> will render <table> tag on the client-side, but you can access it in the code-behind through its ID. The structure looks like this:

<asp:Table ID="MyTable" runat="server">
    <asp:TableRow runat="server" ID="MyRow1">
        <asp:TableCell>Some value</asp:TableCell>
    </asp:TableRow>
</asp:Table>

You can now write something like this in the code-behind:

MyRow1.Visible = False;