I would like to apologize for the inconvenience, however, I'm not sure I completely understand what you mean here.
You can add PayPal from the Payment Tools of the Form Builder page once for each form. However, you cannot add two payment tools on a form. (Unless you use the workaround on this guide: How-to-Create-a-Basic-Multiple-Payment-form)
Meaning, you can add a payment tool on your Registration Form and you can also add a payment tool on your Donation Form.
Is this the same error message you're getting:
This message only appears if you have an existing payment tool on the form. I have checked the form entitled: Online Donation Form. Is this the form you are referring to? I can see that there is already a payment tool added on this form.
If this is not the Form you are referring to, kindly give us the link to your form and we will further investigate. But, if this is the form you are pertaining to, kindly delete the existing payment tool on that form so you can add a new one.
I hope this helps. Let us know if you need any further assistance. Thank you.