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It would depend on what type of Paypal you are using. I've cloned your form and I see that you are using the standard Paypal checkout. In the description it says that you will need to have a Business or Premiere Paypal account, you can log in to your Paypal account to see if you have the same account type.
If the problem is not in the account type, you may try re-adding the Payment tool again and clearing your form's cache. See if that fixes the problem.