System.NullReferenceException: 'Object reference not set to an instance of an object. while working with a word document to edit via c#

I was trying to do a Word Document Editor in c# with the main idea of changing certain fields in a word document. Like for example where the word name, to replace with an actual name that I type in a windows form field. Think of it as a Certificate generator. where i just need to change the Date and name.

But i keep getting an error where the Object reference not set to an instance of an object.

The error im getting is on Word.Range nom = ObjDoc.Bookmarks.get_Item(ref name1).Range;

here is the complete code

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using Word = Microsoft.Office.Interop.Word;

  namespace WindowsFormsApp5
  {
   public partial class Form1 : Form
   {
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        object ObjMiss = System.Reflection.Missing.Value;
        Word.Application ObjWord = new Word.Application();
        string route = Application.StartupPath + @"C:\document.docx";
        object parametro = route;
        object name1 = "name";
        object telephone1 = "telephone";

        
        
        
        Word.Document ObjDoc = ObjWord.Documents.Open(parametro , ObjMiss);
        
        Word.Range nom = ObjDoc.Bookmarks.get_Item(ref name1).Range;
        nam.Text = textBox1.Text;
        Word.Range tel = ObjDoc.Bookmarks.get_Item(ref telephone1).Range;
        tel.Text = textBox2.Text;
        object rango1 = nam;
         object range2 = tel;
         ObjDoc.Bookmarks.Add("name", ref range1);
         ObjDoc.Bookmarks.Add("telephone", ref range2);
        ObjWord.Visible = true;

    }
  }
}

I am completely sure I have the correct route the word document, I also inserted the bookmarks necesary in the place I want to replace the words.

>Solution :

Are you sure that

Application.StartupPath + @"C:\document.docx";

yelds the intended path? It would be something like

"C:\path\to\application\C:\document.docx"

As far as I know, " : " is not even an accepted character for folder names.