When I try to change to Square as my payment method, I am advised that I already have a payment method selected, but I don't

https://www.jotform.co/form/62084310071847?preview

I have checked your form and see that there is a purchase order field on the form which is not allowing to add Square payment. 

You'll need to delete this field if you want to add Square payment to the form. When you delete this purchase order field, please make sure that you are selecting Yes to keep the products so that they can be added automatically to the Square payment field.

Hope this information helps!