Using Alert in Response.Write Function in ASP.NET

Replace:

Response.Write("<script language=javascript>alert('ERROR');</script>);

With

Response.Write("<script language=javascript>alert('ERROR');</script>");

In other words, you're missing a closing " at the end of the Response.Write statement.

It's worth mentioning that the code shown in the screenshot appears to correctly contain a closing double quote, however your best bet overall would be to use the ClientScriptManager.RegisterScriptBlock method:

var clientScript = Page.ClientScript;
clientScript.RegisterClientScriptBlock(this.GetType(), "AlertScript", "alert('ERROR')'", true);

This will take care of wrapping the script with <script> tags and writing the script into the page for you.