Since Mac OS users can update there OS version to 10.12 (sierra), there are problems with the Citrix Receiver.
When you connect with your storefront, you can login, but when you choose a desktop or app the Citrix Receiver stops with the following error: Initialization Failed, Contact your help desk for assistance.
The problem with this is that Apple changed some code for API calls, and with that change the Citrix Receiver gets wrong parameters back and ends with this error.
To work around this problem, you can use the Citrix Receiver as Extension in the Google Chrome browser.
Start the Google Chrome Browser, add the extension, start the extension and enter the address to your storefront.
Maybe not a workaround for everyone, because your NetScaler/storefront configuration may differ, but it’s worth the try.
On september 20, Citrix released a new version with Mac OS Sierra support of there Citrix Receiver for Mac 12.3. You can download the version here.