[Solved] Google recaptcha V2 not working after PostBack (Button Click) in ASP.Net

VerifyCaptcha not needed in the url.

It needs to be set in the Ajax url.

url: "Default.aspx/VerifyCaptcha",

Refer below code.

HTML

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<script type="text/javascript" src="https://www.google.com/recaptcha/api.js?onload=onloadCallback&render=explicit"
    async defer></script>
<script type="text/javascript">
    var onloadCallback = function () {
        grecaptcha.render('dvCaptcha', {
            'sitekey': '<%=ReCaptcha_Key %>',
            'callback': function (response) {
                $.ajax({
                    type: "POST",
                    url: "Default.aspx/VerifyCaptcha",
                    data: "{response: '" + response + "'}",
                    contentType: "application/json; charset=utf-8",
                    dataType: "json",
                    success: function (r) {
                        var captchaResponse = jQuery.parseJSON(r.d);
                        if (captchaResponse.success) {
                            $("[id*=txtCaptcha]").val(captchaResponse.success);
                            $("[id*=rfvCaptcha]").hide();
                        } else {
                            $("[id*=txtCaptcha]").val("");
                            $("[id*=rfvCaptcha]").show();
                            var error = captchaResponse["error-codes"][0];
                            $("[id*=rfvCaptcha]").html("RECaptcha error. " + error);
                        }
                    }
                });
            }
        });
    };
    $(function () {
        $("[id*=txtCaptcha]").val("");
    });
</script>
<div id="dvCaptcha">
</div>
<asp:TextBox ID="txtCaptcha" runat="server" Style="display: none" />
<asp:RequiredFieldValidator ID="rfvCaptcha" ErrorMessage="Captcha validation is required." ControlToValidate="txtCaptcha"
    runat="server" ForeColor="Red" Display="Dynamic" />
<br />
<br />
<asp:Button ID="btnSubmit" Text="Submit" runat="server" />

Code

C#

protected static string ReCaptcha_Key = "<RECaptcha Site Key>";
protected static string ReCaptcha_Secret = "<RECaptcha Secret Key>";

[System.Web.Services.WebMethod]
public static string VerifyCaptcha(string response)
{
    string url = "https://www.google.com/recaptcha/api/siteverify?secret=" + ReCaptcha_Secret + "&response=" + response;
    return (new System.Net.WebClient()).DownloadString(url);
}

VB.Net

Protected Shared ReCaptcha_Key As String = "<RECaptcha Site Key>"
Protected Shared ReCaptcha_Secret As String = "<RECaptcha Secret Key>"

<System.Web.Services.WebMethod()>
Public Shared Function VerifyCaptcha(response As String) As String
    Dim url As String = "https://www.google.com/recaptcha/api/siteverify?secret=" & ReCaptcha_Secret & "&response=" & response
    Return (New System.Net.WebClient()).DownloadString(url)
End Function

Screenshot