CKEditor scayt Plugin module
Without seeing the error it's difficult to know, but you do at least need to extend CKEditorPluginBase rather than PluginBase:

use Drupal\ckeditor\CKEditorPluginBase;

class Scayt extends CKEditorPluginBase implements CKEditorPluginInterface, CKEditorPluginButtonsInterface {


Otherwise the required interface methods won't be satisfied.