Adobe has changed it's policy for which older versions are available for download via the Creative Cloud desktop application. Please see this page for more information:
IMPORTANT: Adobe's Changes to Creative Cloud - May 2019
To install Adobe InCopy CC2018 (which is version 13) you will need a Creative Cloud account (with a paid subscription for InCopy) as well as the "Creative Cloud Desktop App". For more information about Creative Cloud licensing please see the information here. The "Creative Cloud Desktop App" can be downloaded here.
Once the "Creative Cloud Desktop App" is installed you will need to download InCopy CC2018 using the direct download link (hosted by Adobe)