Is it possible to add a Paypal button using a source code?

I've have authenticated paypal using API credentials but received an error message stating that I am not setup to receive online payments when filling out the form although Paypal indicates otherwise. Please help.  

I am afraid using the source code will not work. You will have to use the Paypal form integration, to add Paypal button. 

This link contains comprehensive article with screenshots, on how to integrate Paypal on your form.

https://www.jotform.com/help/50-PayPal-Forms

I'm sorry but it is not possible to add a Source Code directly on your form. If you want to add Paypal button on your form, you need to integrate your form with PayPal. Here's a guide on How to Integrate Form with Paypal. If you have successfull integrate your form with PayPal, the users will be able to be redirected to Paypal checkout page if they have clicked the Submit Button on your form.

Here are some guides too that will help you on how to set it up on your form.

Setting Up Your First Order Form

PayPal Forms

Regards.

I've tried the integration process and entered API credentials. How do I redirect clients to PayPal after clicking submit?
Sent from my iPhone 7
...

After the users filled out the form and clicked the Submit button they will be automatically redirected to PayPal Checkout Page.

You can also test your PayPal Payment Form, to see how it works on your form. Please refer to this guide on How Test PayPal Payment Form.

Regards.

Thank you for the assistance. Now that the form has been integrated, at
which point is it considered a submission? For example, does my customer
have to complete a paypal transaction or does the submission occur after
the client click submits and move to the paypal website?
...

It is considered as submission when the Paypal payment is completed. Paypal will notify you about the payment, and your form will also notify you about the submission, see to it that the notification is configured in the form.

When the customer fails to send the payment, the submission will be added to your incomplete payment list. Incomplete means no payment has been made. For incomplete payments, the following articles will help you fully understand it:

https://www.jotform.com/help/135-How-to-View-Incomplete-Payments

https://www.jotform.com/help/272-Paypal-Incomplete-Payments-FAQ