If you get the error message on your Budget Review page shown in the screenshot below, then here is what you need to do to fix it.
You get a negative budget because you have too much of your budget allocated to your Ad campaigns.
You need to edit your ad campaign budget allocation by the amount of money you are over budget.
In the case of the screenshot below, they are over budget by $28.
So, they would need to lower the budget of one of your campaigns by $28.
Click here to learn how to edit your budget.