Requirements use the word “must” to define something you are required to do. If you cannot fulfill the requirements for an action then you cannot take that action. For example, a card might say “Your opponent must pay 2 for each attacking party they form.” In that case, your opponent could not form a new attacking party unless they could meet the requirement by paying 2. (For more on requirements, see What If More than One Requirement Exists for the Same Action?)