You have not configured a default database for Datasource mongo_db

[angelxmoreno]

However:

'Datasources' => [
        'mongo_db' => [
            'className' => 'CakeMonga\Database\MongoConnection',
            'dns' => env('MONGODB_URL', 'mongodb://localhost:27017/app')
        ],
        'default' => [
            'className' => 'Cake\Database\Connection',
            'driver' => 'Cake\Database\Driver\Mysql',
            'persistent' => false,
            'host' => 'localhost',
            ...
        ]
];