Pleasehelp,
I replied your other posts regarding the use of modals. Again, Whenever you see the following modals /helping verbs used, even in their negative form, you will find the adjacent verb remain in its basic form. i.e. He must understand the consequences.
Does / do/ did/ should/ shall/ may/ might/ will/ would/ can /could /must etc..
This is the hard rule. So for your question: But shouldn't it be has instead of have since the sentence is singular?
The answer is "no" because "doesn't" already satisfied the 3rd person singular requirement.