Uncaught ReferenceError:go is not defined

<!DOCTYPE html>

<html lang="en">

<head>

  <meta charset="UTF-8">

  <meta name="viewport" content="width=device-width, initial-scale=1.0">

  <script type="text/script" src="go.js"></script>

  <title>GOJS</title>

</head>

<body onload="init()">

  <div id="myDiagramDiv"></div>

  <script>

    function init() {

      if (window.goSamples) goSamples();  // init for these samples -- you don't need to call this

      var $ = go.GraphObject.make;  // for conciseness in defining templates

      myDiagram = $(go.Diagram, "myDiagramDiv",  // create a Diagram for the DIV HTML element

        {

          "undoManager.isEnabled": true  // enable undo & redo

        });

      // define a simple Node template

      myDiagram.nodeTemplate =

        $(go.Node, "Auto",  // the Shape will go around the TextBlock

          $(go.Shape, "RoundedRectangle", { strokeWidth: 0, fill: "white" },

            // Shape.fill is bound to Node.data.color

            new go.Binding("fill", "color")),

          $(go.TextBlock,

            { margin: 8, font: "bold 14px sans-serif", stroke: '#333' }, // Specify a margin to add some room around the text

            // TextBlock.text is bound to Node.data.key

            new go.Binding("text", "key"))

        );

      // but use the default Link template, by not setting Diagram.linkTemplate

      // create the model data that will be represented by Nodes and Links

      myDiagram.model = new go.GraphLinksModel(

        [

          { key: "Alpha", color: "lightblue" },

          { key: "Beta", color: "orange" },

          { key: "Gamma", color: "lightgreen" },

          { key: "Delta", color: "pink" }

        ],

        [

          { from: "Alpha", to: "Beta" },

          { from: "Alpha", to: "Gamma" },

          { from: "Beta", to: "Beta" },

          { from: "Gamma", to: "Delta" },

          { from: "Delta", to: "Alpha" }

        ]);

    }

  </script> 

</html>

I am using this code getting this error :-
Uncaught ReferenceError: go is not defined
init file:///E:/GOJSDEMO/index.html:15
onload file:///E:/GOJSDEMO/index.html:1

Your <script> element is expecting the go.js file to be in the same folder as the index.html file. But it is not there.

You can either use a relative path to refer to where the file actually is, or you could copy the file there, or you could load the file from a CDN.
https://gojs.net/latest/learn/index.html
https://gojs.net/latest/download.html