When you first start a lesson on the app, you will be asked if you would like to practise speaking. If no, tap on No speaking. If yes, tap on Practise speaking. This will then allow access to the microphone and activate the speech recognition feature.
If you choose No speaking but later change your mind, this can easily be changed. Just tap on the microphone icon at the bottom-left corner of any lesson, and you can turn the speech recognition feature on and off.
If you are having problems with the app recognising your voice, tap on Profile > Settings > Scroll down to and tap on Microphone, and then adjust the microphone sensitivity level.