Which Android app are you using?
What phone/device are you using, and which Android version do you have installed?
Some older phones do not support speech recognition.
You can also select the "Microphone Config" menu to test the issue and try switching to the Google speech recognition option. Ensure you have the Google app installed, and have speech recognition enabled on your phone. Also some speech recognition requires you have an Internet connection, so ensure you are connected.
Also try rebooting the phone, and ensure you accept the microphone permission request.