It is important to press and hold the mic button for the entire duration of the speech. See article on Introduction to your Control Center remote.
Internet connectivity is required for Control Center to function properly. Voice search depends on internet access. Please verify Control Center is connected to the Internet and that you are able to retrieve a webpage on your laptop or phone to ensure that data is transferring on your home network. Perform a text-based search function (see article How to search for content on Control Center) and ensure you can see results.
Verify that other operations of the remote are working ok by using the up/down/left/right and Caavo buttons to navigate the user interface on the main unit.
If none of the functions on your remote work, see article on What to do if my Control Center remote is not functioning.
Check if the light on the remote flashes green when you press and hold the MIC button. If you see a green light and are keeping the mic button pressed for the entire duration of the voice command, the TV display should first show "Listening" and then show the text string of your search. If the TV display shows "Listening" but does not show the text string of your search, reboot Control Center and check your network (see article on What to do if Control Center can't connect to the internet).
If the issue is still not resolved, you may need to talk to your internet service provider or your router manufacturer for possible issues that may involve blocking of certain ports or firewall.