You can download Hotfix #6 from the TIBCO Support Customer Portal Web UI (https://support.tibco.com).
Log in using your username and password for the TIBCO Support Web page.

  Select Download
  Select Hotfixes
  Select AvailableDownloads
  Select Mainframe
  Select Substation
  Select 2.12.1
  Select hotfix-6

You should then be able to see and download the files.
TIB_substation_2.12.1_zos.sxs.hotfix_#6.zip
TIB_substation_2.12.1_readme_hotfix.txt

Please note: Non-DFSMSdss installation packages are available by request only.
Listed below is a summary of updates included.
Please refer to the associated readme document for any additional information.

Closed issues in Hotfix #6:

SSES-2105 - High
  
  In z/OS where no Mainframe HUB has been started, the #FINDHUB macro will
  cause an abend due to an invalid pointer locating the CVT in certain
  conditions. This has now been fixed.


SSES-2096 - Normal
  
  Changed messages SXG2131I and SXG2133W to include the current UoW allocation.