Captcha Incorrect even if I put the correct letter

I have been trying to use captcha but on validation it brings error saying it not correct.

In the model here is my code:

public $verifyCode;

 public function rules()


        $rules = parent::rules();

        $rules[] = ['location','string','max'=>50];


        return $rules;


In view :

<?= $form->field($model, 'verifyCode')->widget(Captcha::className(), ['captchaAction' => ['/site/captcha']

]) ?>

I attach the image of the captcha field in the browser after putting filling it

Is there Any problem with my code?