How can I add purchase order field?

After I created my form, I discovered I needed to add a purchase order payment method field but it won't let me.  It says I can only have one payment field per form, but I don't have any other payment fields.  What am I doing wrong?  Thanks.

I checked your form and I can see that there is a Purchase Order field already. Please take note that you can only have 1 payment field. You need to remove the existing payment field in order to add a new one.

Hope that helps. Thank you.