When trying to update brew cask with the following command:
brew upgrade brew-cask

Did you ever get the following error?
Error: caskroom/cask/brew-cask not installed

The reason for this error: Warning: The default Caskroom location has moved to /usr/local/Caskroom.

Please migrate your Casks to the new location and delete /opt/homebrew-cask/Caskroom,
or if you would like to keep your Caskroom at /opt/homebrew-cask/Caskroom, add the
following to your HOMEBREW_CASK_OPTS:

  --caskroom=/opt/homebrew-cask/Caskroom

For more details on each of those options, see https://github.com/caskroom/homebrew-cask/issues/21913.
Error: This command requires a Cask token

Easiest way is to move with the following comand: mv /opt/homebrew-cask/Caskroom /usr/local.

Casks installed prior to september 2015 will need to be re-installed manually because they were symlinked rather than moved to their destinations.

brew-cask is no longer under brew list. So brew upgrade brew-cask will no longer work and give you the error Error: caskroom/cask/brew-cask not installed. But don’t worry. It’s now implemented as a homebrew external command called cask.

Resource

https://github.com/caskroom/homebrew-cask