CF7 Forms - Submit button disablement

Question

Hello,

In my CF7 form I have the following code to enable the Submit button (Id=send):

  <script type="text/javascript">
    window.onload = function() {
    document.getElementById('enviar').disabled=false;
  }
  </script>

   .
   .
   .

   <strong>Confirmo que li e  aceito as condições de participação [acceptance aceito id:aceito]</strong></p>      
   <p></p>
   [submit id:enviar "enviar"]

The accepting field (Id=accepted) is connected by default but clicking on the Submit button displays the following message:

"You must accept the terms and conditions before sending your message."

And if you turn off the accepting field, the Submit button is automatically disabled. Does anyone know why this happens? I seek help...