Both Client Key and Server Key are part of your API/SDK keys which is a unique identifier that authenticates requests associated with your profile for usage and billing purposes. To know more about this and how to get them from your dashboard, you may check out our "Authentication/Integration/API Keys" solution article.
As shown below, here's a simple table that indicates to you when to use Server Key and when to use your Client Key
|Server Key||Client Key|
|Other eCommerce Plugins||✔||❌|
|PT2 API Endpoints||✔||❌|