what am i doing wrong when i turn off touchscreen on chromebook?

That flag trick works on a personal Chromebook. If yours came from school or work, the flags page is usually locked by whoever manages it, so you won't get it to stick whatever you do. Only option there is asking the IT people.

There isn't a normal setting for it, which is why you can't find one. Type chrome://flags in the address bar, search for the debugging keyboard shortcuts flag, set it to enabled and restart. After that, search + shift + t toggles the touchscreen off and back on. Worth knowing it's a developer option rather than a supported feature, so it does occasionally vanish after a big update and you have to set the flag again.