This error is caused by a setting in your PayPal account that only works if you are using PayPal's own payment buttons, rather than buttons provided by an independent service such as E-junkie. To fix this error:
- Log into your PayPal account;
- Go to Account Settings (⚙️ gear icon) > Website Payments > Website Preferences;
- Scroll down to Encrypted Website Payments;
- Set Block Non-encrypted Website Payment: OFF;
- Scroll to the bottom of the page and click Save.
Don't worry, your buyer checkouts will still be securely encrypted even with that setting disabled. That setting only prevents tampering with PayPal's own purchase button codes, where the item price and other product settings are configured in the button code itself in your page. Theoretically, a crook could copy your page, modify PayPal's button codes in their copy, and use their tampered version of that page to place an order, so PayPal's encrypted-buttons option is intended to foil that technique.
With E-junkie, your prices and product settings are configured in your Seller Admin, and our button codes only contain a pointer to look up those settings in our database. This makes our button codes immune to the sort of tampering that could happen with PayPal's own un-encrypted buttons, therefore PayPal's Encrypted Website Payments setting is unnecessary to prevent tampering in our case.