Authentication (& authorization)
Each request to a service must be accompanied by an authentication key. This is necessary to properly authenticate the user of the API. The authentication key can be found in the user account in the section Settings->API access in shop admin.
Depending on the operation and type of data, each request to a service must also have proper permissions. Permissions are divided into type of action (create, retrieve, update & delete) and type of data (customer, product, order, etc.).
Permissions are assigned to user groups, and users are assigned to user groups. This is also done in the section Settings->API access in shop admin.
If you, as a developer, do not have access to this site, you need to contact the shop owner to obtain an authentication key with sufficient permissions.