It depends on the plan you are on. Master and Hero users can create an unlimited number of rules, but Apprentice users cannot create more than 3 rules in total regardless of the type. This means that if you already have 2 field rules, you cannot create more than one, be it a field rule, a confirmation rule or a form rule.
Lastly, free accounts can not benefit from this feature, unless they upgrade to a paid plan. Follow our features matrix for more information.
Try using multiple conditions in the same rule or setting up rules that affect more form fields with the same rule to achieve the results you are after if using Apprentice or a higher plan. Or upgrade your account at any time to create as many rules as you want.