ConfirmFormBase to FormBase
There is no FormBase::buildForm() method. FormBase implements FormStateInterface, and the method FormStateInterface::buildForm() exists. So when you extend FormBase, you must implement buildForm(), however you do not/cannot call parent::buildForm(). Instead, you must build the form array yourself, and return that.

This differs from ConfirmForm which has a buildForm() method, which is why when you extend ConfirmForm you can call parent::buildForm().