ScriptManager not calling Js function

I have a server control page where the input is entered and when button is clicked I want to trigger function in .aspx and again continue the C# code written below the script manager.

Now the thing is that I used ScriptManager.RegisterStartupScript(this, GetType(), "", "trackButtonClick()", true); and even I used ClientScript for checking but still the function is not being called or run written in .aspx

The below codes after ScriptManager.RegisterStartupScript(this, GetType(), "", "trackButtonClick()", true); run smoothly but not the JS function declared in .aspx

I have no AJAX POST calls or any other. The page is completely server side control

I tried every solution can anyone check and say what is the issue here as nothing displays in console or anywhere?

Can anyone Help?

protected void btnPay_Click(object sender, EventArgs e)
{
  // other codes

  ScriptManager.RegisterStartupScript(this, GetType(), "", "trackButtonClick()", true);

  // other codes
}

This is the .aspx

<script type="text/javascript">
  function trackButtonClick() {
    console.log('trackButtonClick function executed');
  }
</script>