If you want to disable the group, you need to tell what this.Personal
is, now you have just declared it as a FormGroup, nothing else.
So you can do it after building the form:
this.Personal = this.myForm.get('Personal')
Then you can just disable it with:
this.Personal.disable();
DEMO: http://plnkr.co/edit/QAhY6A9950jqrgzvjVKu?p=preview