From the knowledgebase:
https://www.steinberg.net/nc/en/support/knowledgebase_new/show_details/kb_show/workaround-osx-enable-the-audio-unit-of-halion-symphonic-orchestra.html
Workaround OSX: Enable the Audio Unit of Halion Symphonic Orchestra
At the moment the Audio Unit component of the new Halion Symphonic Orchestra is not working as expected.
Therefore the AU component cannot be loaded into a host application.
Please download the file “HSO AU.dmg” and run this tool.
After the Installation is completed, the AU Plug-In will work as it is should.
Do this after you have run the regular installer.