CSOM : Server Unauthorized Access Exception: Access Denied.

My suggestion, you're using SharePoint On-Premises.

For On-Premises credentials should be defined in the following way:

using System.Net;
using Microsoft.SharePoint.Client;

using (ClientContext context = new ClientContext("http://sp-onprem/")) {

    context.Credentials = new NetworkCredential("user", "password", "domain");
	
    // CSOM code

}

SharePoint Online requires the secure string as a password:

using (ClientContext context = new ClientContext("https://contoso.sharepoint.com/")) {

    string pwd = "password";
    SecureString password = new SecureString();
    foreach (char c in pwd.ToCharArray()) 
    {
        password.AppendChar(c);
    }
	
    context.Credentials = new SharePointOnlineCredentials("user", password);
	
    // CSOM code

}

Also, I would recommend to figure out what auth scenario your instance is configured to support.