If you get a notification that a Snap package like Slack needs to be closed for a pending update, it's not immediately obvious what Ubuntu wants you to do:
It needs the slack app to be closed so it can install an update that is pending. What you'll need to do:
Quit
sudo killall snap-store
sudo snap refresh
to perform the update manuallyEspecially for long running applications like Snap, Firefox or Chromium, it's a good idea to force the update right now and make the notification go away.