I cannot add Paypal in the form because it says that I already have one

Cannot add PayPal because it says that I already have a payment option on the Membership Form, but I don't. 

You can only add one payment integration in the form. You have the "Membership Type" in the form which is a Purchaser Order Tool. You have to remove that in order to be able to add Paypal. 

Sorry. I am confused. I need the three types of memberships to subscribe to and then payment by PayPal. What am I missing?

...

We have different payment integration as you can see under the "Payment Fields" section, you can only add ONE of it in your form.

 

In your case, you have added "Purchase Order" in your form, you cannot add another payment field like Paypal on the same form:

 

Usually the Purchase Order payment field is designed for users who would still like to setup product listing but the payment method to charge their users is manual, either they ask their user to pay via check, wire transfer, bank transfer and the likes.

If you wish to change the "Purchase Order" to a Paypal payment field, simply delete the purchase order. Here's a quick step by step:

1. First, click the "Delete" button on the payment field.

2. It will then ask you if you want to keep your products, click "Yes" if you like to:

3. After than, drag the "Paypal" payment field you want to add in your form. Here's a screencast:

 

 

To learn more about setting up your order form, you can check this guide: 

https://www.jotform.com/help/47-Order-Form-Basics 

https://www.jotform.com/help/49-Order-Form-Types 

https://www.jotform.com/help/165-Setting-Up-Your-First-Order-Form 

 

If you want to setup multiple payment methods, then you can check this guide: 

https://www.jotform.com/help/182-How-to-Create-a-Basic-Multiple-Payment-form 

 

I hope that helps.