Unresolved reference: RtcEngine, Unresolved reference: IRtcEngineEventHandler, Unresolved reference: CHANNEL_PROFILE_COMMUNICATION

Errors

Unresolved reference: io
Unresolved reference: RtcEngine, 
Unresolved reference: IRtcEngineEventHandler, 
Unresolved reference: CHANNEL_PROFILE_COMMUNICATION

What I tried
https://docs.agora.io/en/Voice/start_call_audio_android?platform=Android