Mago requires only 3 things to create a new contact - First name, last name, and email address. This is to help prevent confusion in the system with contacts that have same first or last names, and to facilitate future communication with them.
When you build a form in Mago, we start you out with the required fields already on the list. From there, you can add whatever fields you would like.
If you remove one of the required fields, you will get an error upon testing your form that looks like this:
This means one of those fields is missing. You'll just need to edit your form and put it back. Once that is done and your form has first name, last name, and email address fields in place, it should work. If it doesn't, it's time to submit a support ticket.