,Marketplace account deletion endpoint validation failed

I don't know what to do more than this

Help please

Here's my code

Public Class challengeResponseOBJ
    Public Property challengeResponse As String
End Class

 Public Function checkEbayRequestAndReturnResponse(request As HttpRequest) As challengeResponseOBJ
        Dim method = request.HttpMethod
        Dim response As String
        If method = "GET" Then
            Dim challengeCode = request.QueryString("challenge_code")
            If String.IsNullOrWhiteSpace(challengeCode) Then
                Return Nothing
            Else
                Dim hash = EbayNotificationCheck(challengeCode)
                Return New challengeResponseOBJ With {.challengeResponse = hash}
            End If
        Else
           return nothing
        End If
    End Function

 Public Function EbayNotificationCheck(challenge_code As String) As String
        Dim verificationCode = "<verficationCode>"
        Dim url As String = "<myUrl>"
        Dim sha256 As SHA256 = SHA256Managed.Create()
        Dim stringToEncode = challenge_code + verificationCode + url

        Dim bytes As Byte() = Encoding.UTF8.GetBytes(stringToEncode)
        Dim hash As Byte() = sha256.ComputeHash(bytes)
        Return BitConverter.ToString(hash).Replace("-", String.Empty).ToLower()
    End Function