So, in case someone else gets stuck on this, the problem for me turned out to be that although both sandbox accounts were correctly set up in my developer account and linked to my sandbox account (ALL of which needs to be done from your developer account), I hadn't entered the correct API details in my website. Here is the correct way to to it (which worked for me): On the website, under WooCommerce > Settings > Payments Check the Enable PayPal sandbox check box Enter the sandbox seller email address in the PayPal email and Receiver email text fields (i.e. paypal-facilitator @ absurdpublications.com) For API credentials, go to your developer account, open the SANDBOX Accounts page, click on the Profile link under the buyer account (i.e. paypal-facilitator @ absurdpublications.com) (the link only works intermittently on my system), click on the API Credentials tab, copy the Username, Password and Signature listed under the NVP/SOAP Sandbox API Credentials heading, into the relevant fields in the API credentials area on your website (they should be labelled Sandbox API username, Sandbox API password, Sandbox API signature) Click on Save changes Good luck!!
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