Updates are paused while an issue is investigated, see the weekly update. Some users have received the update before the pause (I know of one who is using the update without problems) but as far as I can see, none of the builds have been released to the download pages in the documentation yet.
Is the tab booting to 0.18-r ok?
Maybe check in the updater app if the download for 0.21-r is still present. Check to make sure it is the correct version by right click on it and copy paste the url to somewhere you can read it.
I don’t know if it’s possible for the pausing of updates to affect your download so that it would install but not boot.