How can I choose Purchase Order only as payment option?

I can't seem to add Purchase Order as a payment option. The only clickable option is the PayPal?

Unfortunately, you can have only on payment field per form. In order to add Purchase Order field, you have to delete other payment fields in your form as you can see below:

However, these guides may help you:

How to Create a Basic Multiple Payment Form

How to Update the Multiple Payment Form

Also, there is a guide about Purchase Order if you want to learn more about this field:

How to Create Payment Form with Purchase Order