Occasionally (not all the time), when invoking the Google Assistant on my Pixel 6 phone, I get the exact message "For that, you'll have to turn on Wi-Fi and connect to a network."
Online, I have zero results for that message when searching for it, which seems plain weird. I've tried giving google assistant any permissions it might not have had out of the box, alas, the problem persists. A reboot usually does fix it though. Still, a nuisance. Cellular data is always on, and I live in San Diego.