How do I make my checkboxes link up with my drop down list?

I'm wanting to make an order form with multiple check boxes within a group box. However I need to link them up so that an error message occurs if the checkbox is ticked but no quantity has been chosen.

How should I go about this?

e.g. ticking the box for pepperoni pizza but not choosing a quantity limited from 1-10.

Have tried to do multiple things but nothing is working.