add.ctp

   <?= $this->Form->create($contatcs) ?>

       <fieldset>  
      <div class='input-list style-2 clearfix'>   
        <?php


            echo $this->Form->input(('nome'), array('class' => 'form-control'));
            
            echo $this->Form->input(('Email'),array('class' => 'form-control','empty' => 'entrada' ));

            echo $this->Form->input(('mensagem'),array('type' => 'textarea','class' => 'form-control','empty' => 'entrada2' ));

       
        ?>  
           </div>
    </fieldset>
    <br/>
    <?= $this->Form->button(__('Enviar Mensagem'), 
    array('class' => 'btn btn-primary button-marques', 'title' => 'Enviar Mensagem')) ?>
    <?= $this->Form->end() ?>

ContatcsCell.php

<?php

class ContactsCell extends Cell
{
    protected $_validCellOptions = [];

    public function display()
    {
    	<?= $this->cell('Contact::add')   ?>

    }
}

Home.ctp

<? = $cell = $this->cell('Contatcts'); ?>

you've misspelled contacts in both your home.ctp and add.ctp (form->create) and your Cell display method should act exactly like a Controller action, so ..

<?php
class ContactsCell extends Cell
{
    protected $_validCellOptions = [];
    public function display()
    {
        $contact = $this->Contacts->newEntity();
        $this->set(compact('contact'));
    }
}

   <?= $this->Form->create($contact, ['controller'=>'Users', 'action'=>'add']) ?>
       <fieldset>  
      <div class='input-list style-2 clearfix'>   
        <?php
            echo $this->Form->input(('nome'), array('class' => 'form-control'));
            echo $this->Form->input(('Email'),array('class' => 'form-control','empty' => 'entrada' ));
            echo $this->Form->input(('mensagem'),array('type' => 'textarea','class' => 'form-control','empty' => 'entrada2' ));
        ?>  
           </div>
    </fieldset>
    <br/>
    <?= $this->Form->button(__('Enviar Mensagem'), 
    array('class' => 'btn btn-primary button-marques', 'title' => 'Enviar Mensagem')) ?>
    <?= $this->Form->end() ?>

<?= $cell = $this->cell('Contacts'); ?>