First of all, the Web API plugin has to be installed on the nopCommerce. Only registered customers (in nopCommerce) with both keys (public and secret key) have got an access to the API. Web API plugin uses ACL rules in nopCommerce.
Register customer for Web API (user should exist in nopCommerce as a registered user). Go to Plugin List -> (on WebAPI plugin) Configure. First you will be able to see configuration of the token.
Below you may add customer for API
Since nopCommerce 4.20 there is only email and password for authenticating the user.