How do I stop 'beep' on error sound?

Big Sur.

When I make an error (e.g. cmd-I on a folder to look for info when Big Sur cannot do that) ... the Mac beeps. I would like to stop this 'beeping on error' behaviour.

It is nothing to do with the start-up tone ... that went within 10 seconds!!! When I look at the same system preferences that disabled the start-up tone there is a box labelled 'Play user interface sound effects'. This is off. AFAIK I should now hear no system sound effects.

Where else should I be looking?

BTW. I am still hoping for a better community interface .. and I am hoping that I am posting in the correct place. First two words 'Big Sur'. Recommendations ... 2 phones, a watch, High Sierra and Sierra (!!!) Try choosing a device and MacBookPro, air etc. are not even on the list! Must expand the list! System is a joke. /rant