I can't properly set up PayPal API credentials

I'm trying to setup PayPal payment gateway, but can't get it work. I get the following error when I'm trying to buy a product in the shop:
10002 - Security header is not valid

I can't find the option to request/view signature at the PayPall API Access page, so I'm trying to use Fingerprint instead of this, is this correct?
I'm sure that my credentials are correct and that these credentials are live. I've tried to regenerate credentials few times, but this doesn't help.