If your Google speaker is saying "I didn't understand that", it may not be recognizing the correct command to access Helping Hand,
When this happens, it is worth checking your Google activity to make sure your device is understanding you well.
Open the Google Home app
Tap on your profile icon and then click "Manage your Google Account"
Under Data & Personalization, scroll down to My Activity
Here you can see what Google recognized as the command. If you are experiencing problems with your Google speaker recognizing you often, ensure that you are in a quiet environment.