Heya – I’m running into the same issue here.
PayPal Login FAILED – API Error 10002 – Security error
Pretty sure I’m entering everything correctly but I’m still running into trouble. I tried to setup debug mode as you suggested here but the “Show PayPal IO” feature is not available in the debug config page.
I did notice a difference in terminology between what I see in my paypal account and what the plugin requests. i.e. the plugin asks for an API signature and my API settings in paypal has an API Signature.
Any ideas?