For forms where you have multiple checkboxes that are required to be selected, instead of putting them all in one control, make each required option it’s own Checkbox. This method allows you to make each control required so the users must select the option to submit.
In the example below the single checkbox option is Option 1 and Section A will only show after the user has checked the checkbox. In the rules you will need to add  before .value in your code. In Frevvo, checkboxes are treated as arrays, the  is used to specify the array number in the sequence and because you only one option in your control, the sequence number is 0.