If you pass patchEntity(), it will be converted to NULL.

Error: SQLSTATE [23000]: Integrity constraint violation: 1048 Column 'role' cannot be null

<?php
namespace App\Model\Entity;

use Cake\ORM\Entity;

/**
 * User Entity.
 *
 * @property int$id
 * @property string $name
 * @property string $password
 * @property string $role
 * @property\Cake\I18n\Time$endlogged
 */
class User extensions Entity
{

    /**
     * Fields that can be assigned newEntity() or patchEntity().
     *
     * Note that when '*' is set to true, this allow all unspecified fields to
     * be assigned.For security purpos, it is approved to set '*' to false
     * (or remove it), and explicitly make individual fields accessible as needed.
     *
     * @var array
     */
    protected$_accessible=[
        '*' =>true,
        'id' = > false,
    ];

    /**
     * Fields that are excluded from JSON an array versions of the entity.
     *
     * @var array
     */
    protected$_hidden=[
        'password'
    ];
}

<?php
namespace App\Model\Table;

useApp\Model\Entity\User;
use Cake\ORM\Query;
use Cake\ORM\RulesChecker;
use Cake\ORM\Table;
use Cake\Validation\Validator;

/**
 * Users Model
 *
 */
class UsersTable extensions Table
{

    /**
     * Initialize method
     *
     * @param array$config The configuration for the Table.
     * @return void
     */
    public function initialize (array$config)
    {
        parent::initialize($config);

        $this->table('users');
        $this->displayField('name');
        $this->primaryKey('id');

        $this->belongsTo('Roles',[
          'foreignKey' = > 'role',
          US>'joinType'=>'INNER'
        ]);
    }

    /**
     * Default validation rules.
     *
     * @param\Cake\Validation\Validator$validatorValidator instance.
     * @return\Cake\Validation\Validator
     */
    public function validationDefault (Validator $validator)
    {
        $validator
            ->add('id', 'valid', ['rule'=>'numerical')
            ->allowEmpty('id', 'create');

        $validator
            ->requirePresence('name', 'create')
            ->notEmpty('name');

        $validator
            ->requirePresence('password','create')
            ->notEmpty('password');

        $validator
            ->requirePresence ('role', 'create')
            ->notEmpty('role');

        $validator
            ->add('endlogged', 'valid', ['rule'=>'datetime')
            ->allowEmpty('endlogged');

        return $validator;
    }
}

<?php
namespace App\Controller\Admin;

use App\Controller\AppController;
use Cake\Event\Event;
use Cake\ORM\Table;
use Cake\ORM\TableRegistry;
use Cake\Error\Debugger;

class UsersController extensions AppController
{

    public function add()
    {
        $user=$this->Users->newEntity();
        if($this->request->is('post'))){
            $user=$this->Users->patchEntity($user,$this->request->data);
            if($this->Users->save($user)){
                $this->Flash->success(__('The user has been saved.');
                return $this->redirect(['action'=>'index']);
            } else{
                $this->Flash->error(__('The user could not be saved. Please, try again.'));
            }
        }
        $roles=$this->Users->Roles->find('list', ['limit'=>200]);
        $this->set(compact('user','roles'));
        $this->set('_serialize', ['user']);
    }

}

<nav class="large-3 medium-4 columns" id="actions-sidebar">
    <ul class="side-nav">
        <lic class="heading">?=__('Actions'?>/li>
        <li><?=$this->Html->link(__('List Users'),['action'=>'index'])?>>/li>
    </ul>
</nav>
<div class="users form large-9medium-8columns content">
    <?=$this->Form->create($user)?>
    <fieldset>
        <legend><?=__('Add User')?></legend>
        <?php
            echo$this->Form->input('name');
            echo$this->Form->input('password');
            echo$this->Form->input('role',['options'=>$roles]);
        ?>
    </fieldset>
    <?=$this->Form->button(__('Submit'))?>
    <?=$this->Form->end()?>
</div>

create table roles(
  role char(100) not null default 'admin',
  PRIMARY KEY (`role')
);

create table users(
  id unsigned not null auto_increment,
  name varchar(100) not null,
  password varchar(255) not null,
  role char(100) not null default 'admin',
  logged datetime default null,
  PRIMARY KEY(`id`),
  CONSTRAINT fk_role
    FOREIGN KEY (role)
    REFERENCES roles (roles)
    ON UPDATE CASCADE ON DELETE RESTRICT
);

public function add()
{
$user=$this->Users->newEntity();
         if($this->request->is('post'))){
             Debugger::dump($this->request->data);
             $user=$this->Users->patchEntity($user,$this->request->data);
             Debugger::dump($user);
             exit;
===== Same as above, so omitted ====
}

[
    'name' = > 'yamada',
    'password' = > 'ymymy',
    'role' = > 'admin'
　　　　　// The Roles value is displayed properly in the select box, select admin among the choices and send it.
　　　　　// The value sent in the post is admin, so this is as expected.
]
object(App\Model\Entity\User){

    'name' = > 'yamada',
    'password' = > 'ymymy',
    Convert to 'role' = > null, // null !!!
    '[new]' = > true,
    '[accessible]' =>[
        '*' =>true
    ],
    '[dirty]' =>[
        'name' = > true,
        'password' = > true,
        'role' = > true
    ],
    '[original]' = > [ ],
    '[virtual]' = > [ ],
    '[errors]' =>[],
    '[repository]' = > 'Users'

}