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If the error persists, I would advise you to contact PayPal Customer Support.There can be multiple reasons for this error message. Unfortunately the message never tells you what is wrong or how to fix it. For example, could be a system glitch, could be an issue with the receiver's account, is your account verified, are any credit cards expired, etc.
If you plan to try again later, before you do, delete your web browser's cookies, temp files and history. Then close the browser, reopen it and retry. If you get the same message, next step would be to contact customer support direct for assistance.