Getting unknown property: yii\web\User::admin

I am quite new to Yii 2 and I am trying to hide some navbar items from users that are not admins. However, I somehow get an error stating that the User model has no such property called admin.

I am confused since the model has an attribute called admin with a boolean value. I have been searching on different websites but I can't seem to find a solution so far. Whenever I login as a User, no matter if it's an admin or not, the navbar items are shown.

Below is what I am trying to do in the main.php

<header>
    <?php
    NavBar::begin([
        'brandLabel' => Yii::$app->name,
        'brandUrl' => Yii::$app->homeUrl,
        'options' => [
            'class' => 'navbar navbar-expand-md navbar-dark bg-dark fixed-top',
        ],
    ]);
    echo Nav::widget([
        'options' => ['class' => 'navbar-nav'],
        'items' => [
            ['label' => 'Home', 'url' => ['/site/index']],
            ['label' => 'About', 'url' => ['/site/about']],
            ['label' => 'Contact', 'url' => ['/site/contact']],
            ['label' => 'Team', 'url' => ['/team/index'], 'visible' => Yii::$app->user->admin ? true : false],
            ['label' => 'User', 'url' => ['/user/index'], 'visible' => Yii::$app->user->admin ? true : false],
            Yii::$app->user->isGuest ? (
                ['label' => 'Login', 'url' => ['/site/login']]
            ) : (
                '<li>'
                . Html::beginForm(['/site/logout'], 'post', ['class' => 'form-inline'])
                . Html::submitButton(
                    'Logout (' . Yii::$app->user->identity->username . ')',
                    ['class' => 'btn btn-link logout']
                )
                . Html::endForm()
                . '</li>'
            )
        ],
    ]);
    NavBar::end();
    ?>
</header>

I am not understanding why Yii::$app->user->adminis not working.

I have tried Yii::$app->user->getIdentity('admin') as well but it's not working either. As I said above, the User model does have an attribute called admin with a boolen value.

Any help is greatly appreciated.