The process cannot access the file

Hi all,
I am using this simple function to encrypt the string
public static string Encryptoin(string EncryptWord)
        {
            if (string.IsNullOrEmpty(EncryptWord))
                return "";
            string KeyFileName = HttpContext.Current.Request.PhysicalApplicationPath + "\\symmetric_key.config";

            SymmetricEncryptionUtility.AlgorithmName = "DES";
            byte[] data = SymmetricEncryptionUtility.EncryptData(EncryptWord, KeyFileName);
            
        ...