calling a web API in event handle is not working,

I've tried to setup a service to call data and also tried to create a method to call data but still facing the same error
I want to select a value from Lookup and on its value change I want to call a web API, I can log the event and edit values in my Form Model but cannot call a API, I'm getting a uncaught error at this.http and http declared in constructor as private Http: HttpClient in both caeses