$this->form->getUser() of type object<yii\web\IdentityInterface> is not a sub-type of object<Da\User\Model\User>. It seems like you assume a concrete implementation of the interface yii\web\IdentityInterface to be always present.
This check looks for parameters that are defined as one type in their type
hint or doc comment but seem to be used as a narrower type, i.e an
implementation of an interface or a subclass.
Consider changing the type of the parameter or doing an instanceof check before assuming
your parameter is of the expected type.
If you access a property on an interface, you most likely code against a concrete implementation of the interface.
Available Fixes
Adding an additional type check:
Changing the type hint: