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. Also, if it's the snap-store itself that needs to be updated, you can't use the GUI updater since it can't be running during the upgrade

Regular Upgrades for apps like Slack

It needs the slack app to be closed so it can install an update that is pending. What you'll need to do:

• Close all of the application's windows
• If it's Slack or another application that continues to run in the background, find the little icon next to the clock, right-click it and select Quit
  • If it's an application running in the background without any visible icon (like snap-store) you can run sudo killall snap-store
• In a terminal, run sudo snap refresh to perform the update manually

Especially 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.

Updates for the Snap store

When you receive the notification that the snap store needs to be updated and click it, you'll find out you can't update the snap store this way and will receive the same notification tomorrow. This is because the GUI application that displays and tries to install the updates is the snap store and it can't update itself while it's running.

The solution is to close the Snap Store window, open a terminal and perform the update there:

Copy$ snap-store --quit
$ snap refresh

It should show a progress bar, indicating it's actually updating.