DropDown binding issue in asp .net

this append child is at the end of the function only

The error message "Uncaught TypeError: Cannot read properties of null (reading 'appendChild')" indicates that the ddlCountries element is null. Make sure the JavaScript code is executed after the ddlCountries dropdown is rendered on the page. Placing the JavaScript code at the end of the page can help ensure that the dropdown element exists before accessing it.

getting error as Uncaught TypeError: Cannot read properties of null (reading 'appendChild')

try this once:

var ddlCountries = document.getElementById('<%= ddlCountries.ClientID %>');
var option = document.createElement("option");
option.text = "Option Text";
option.value = "Option Value";
ddlCountries.appendChild(option);

Tried this as well but it's not working its ext gridview 

Try below code:

// Get a reference to the dropdown element
var dropdown = document.getElementById('<%= ddlCountries.ClientID %>');

// Create a new option element
var option = document.createElement('option');
option.value = 'value1'; // Set the value of the option
option.text = 'Option 1'; // Set the text of the option

// Append the option to the dropdown
dropdown.appendChild(option);

Thanks