Web API

I have created a webapi - Get method, I have created a basic user and added that user in a Group and attached that Group in that WebAPI. Now I used that basic method and called that using PostMan , it works fine.

Now I deleted that basic user from that group & created a API Key and attached that Service Account in that group. 

Used the API Key & Value in PostMan, I am getting an error 

Pls let me know, what is the mistake I am making here. Thanks in advance.