Error: It seems the App source is not there: '/usr/local/Caskroom/multibit/0.5.19/MultiBit.app'
I had this with two apps. It happened because I removed them via the Applications folder, using the finder.

Error: Problems with multiple casks:
darktable: It seems the App source '/Applications/darktable.app' is not there.

I got rid of this message by removing the application from cask:
brew cask remove darktable
==&gt; Uninstalling Cask darktable
==&gt; Purging files for version 3.0.2 of Cask darktable