Class Module does not exist

Question

I'm using ut8ia/yii2-content-module for content managing. but I have bellow error when I want to use it:

Class \ut8ia\contentmodule\ContentModule does not exist

I done as the documentation order.

here is my config in backend:

 'modules' => [
    'content' => [
        'class' => '\ut8ia\contentmodule\ContentModule'
    ],
]

In the vendor\ut8ia\yii2-content-module directory, there is the following class ContentModule:

namespace ut8ia\contentmodule;

use \yii\base\Module;
use ut8ia\contentmodule\models\Content;

class ContentModule extends Module
{
...
}

where is the problem?

Please help me to solve it.