Hi Anna,
I have checked and double checked the API credentials and even entered incorrect ones – in which case it gives me a different error (incorrect details error). That’s the other reason I know I am entering the correct details.
What do you make of the URL query string that is passed to PayPal – it looks incomplete (https://www.paypal.com/cgi-bin/webscr?cmd=_ap-payment&paykey=&Z3JncnB0=)?
When you do a checkout at a fully working wc vendors store, is the query string missing key elements too?