Google might respond with ‘I don’t understand’ if you give a command that it cannot perform. Check whether the command you gave was accomplishable by Google.
- If you are using a Google Home speaker or any smart speaker equipped with Google Assistant, try cleaning the speaker so that the speaker hears the command with maximum clarity.
- Factory reset the device on which the Google Assistant does not understand you.
- If on a smartphone, go to Google Settings, Popular settings and go to Voice Match. Here you can reset your voice alone on the device.