Apple released Mac OS X Sierra on September 20th as a free upgrade through the Mac App Store.
The Citrix Receiver for Mac OS X version 12.2 does not suffer from the SSL issue on Sierra.
Version 12.1 has found to be incompatible with OS X Sierra if you are accessing your StoreFront or Netscaler over SSL.
The error shown on the client computer is 'Initialization failed. Contact your helpdesk for assistance.'
Citrix Support determined that Apple made a change to an API that Receiver was calling when setting up a HDX connection over SSL. The API in question was changed to always return a failure. It had been marked as deprecated and Apple decided to follow through with that deprecation in Sierra and make it always fail.
To address this issue, you can download the new Citrix Receiver for Mac from the Citrix website: https://www.citrix.com/downloads/citrix-receiver/mac/receiver-for-mac-latest.html
This last version brings full OS X Sierra support.