"ClickOnce does not support the request execution level 'requireAdministrator.'"
Edit: This comment gives a good answer, too.
Click once appears to get enabled whenever you click "Publish", whether you want it to or not! If you are using "requireAdministrator" then it appears that you cannot use ClickOnce, and therefore cannot "Publish" your project.
Original:
Turns out that under the Security tab, "Enable ClickOnce security settings" was checked. Even though I didn't check it.Anyway, unchecking that stopped ClickOnce giving me errors. That took a while to find...
