Unable to add Standard Paypal field

I am trying to integrate standard paypal to in this form and I cannot seem to do it.

Any idea why?

I can see you're already using a Purchase Order field (which is also considered a payment field):

This is the reason why you cannot add a Paypal field as there cannot be two payment fields on a single form.

I would recommend deleting the existing Purchase Order field to be able to add a Standard PayPal field. No need to worry about the products as they will be carried over to the Paypal field, so re-creating them is no longer necessary.