Based on the error I was able to reproduce it looks like your PayPal account is setup to only allow encrypted payments. You can disable that by following this guide:
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Log in to your PayPal business account at www.paypal.com .
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Click the profile icon (
) on the top right side of the page. From the Business Profile menu, select Profile and Settings.
Note: If you do not see the profile icon on the top right, select Profile which appears in the top menu when the My Account tab is selected.
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Click My selling tools on the left side of the page, then click the Update link in the Website preferences row. The Website Payment Preferences page opens.
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Scroll down the page to the Encrypted Website Payments section and disable the block.
