Can I add a payment field on a free form?

Yes, please note that on a free account you can receive up to 10 payments per month. You may review this guide regarding payment forms: 

https://www.jotform.com/help/47-Order-Form-Basics 

https://www.jotform.com/help/49-Order-Form-Types 

https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form