I just encountered this problem and did find a solution.
- go to Citrix website and download 12.9.1.
- also download the auto updater.
- open Citrix installer and choose the 'Remove Citrix Receiver' option to remove all elements of previous version.
- once completed, choose 'Install Citrix Receiver' to re-install. It will open as 'Citrix Workspace'
- Once completed, open the AutoUpdater download and install.
Apple released OS X El Capitan on September 30th 2015, which may require you to update your installed version of Citrix Receiver. Citrix Receiver for Mac 11.9 is not.
![Citrix receiver for apple iphone Citrix receiver for apple iphone](https://www.citrix.com/blogs/wp-content/uploads/2017/09/Picture1-2-720x618.png)
![Citrix receiver for mac mojave Citrix receiver for mac mojave](http://i.ytimg.com/vi/u01_IQSouJk/maxresdefault.jpg)
Make sure to address the appropriate option under Safari/preferences/website to allow use of Citrix Workspace.
Citrix now works as before.
Good luck. It certainly caused me a lot of grief these past two days!!