Purchase Order payment option does not appear to be active

For me, the Purchase Order button you suggested does not appear to be active. How do I activate this button?

"This method would require the payment to be submitted separately or offline."

What does this mean?

You can add only one payment field to a form. Considering that your form already has the PayPal payment field you are unable to add the Purchase Order payment field. 

You need to delete the PayPal payment field to be able to add a different payment field. 

"This method would require the payment to be submitted separately or offline."

It means that the payment will not be processed with the Purchase Order payment field and that you can process it offline. 

Purchase Order payment tool allows you to create payment forms, order forms or purchase order forms without payment vendor integration.

Thank you. It was really helpful information.