There are 2 main problems in your code.
The first one is, that the array with url should have format of ['controller-id/action-id'] but your url has format ['ControllerClass/actionMethod'].
The second one is the position of action in your array. As Sfili_81 mentioned in his answer https://stackoverflow.com/a/58797242/11977068
So the correct code should look like:
$form = ActiveForm::begin([
'action' => ['site/upload-image'],
'method' => 'post',
'options' => ['enctype' => 'multipart/form-data'],
]);
Also since yii 2.0.8 there is no need to explicitily use enctype option if you are using ActiveForm. Calling $form->field(...)->fileInput() will ensure the correct enctype.