[Android Central Question]

I give a command of "Call David", as an example, and usually it would respond "Home or Mobile" and then "Calling David". Now, when I give the command, "Call David", or anyone else, I get, "Not sure how I can help with Call David."

[Mooncatt]

Check that you have internet access.
Check that access to contacts is granted in permissions of Google Play Services (denying this one caused this problem for me), the Google app itself, and Google Contacts Sync. And check that the contact you are trying are both in your contact list on the phone AND on the Google serves via your account on their website.

Even if a contact is on your phone, the Assistant will reference the Google servers when using voice commands to dial. If the app cannot access the online server, you'll get these sorts of errors.