Thanks very much for the interest in Hauptwerk.
The current version of Hauptwerk is v6.0.2 (but platform compatibility was the same for v5). The 'Prerequisites' link near the top-right of this page list the macOS versions that are supported:
https://www.hauptwerk.com/requirements/
Here's the relevant excerpt:
On the Mac platform, Hauptwerk is compatible with, and fully supported on, Macs with Intel CPUs running either:
• macOS ‘High Sierra’ 10.13.6 and above.
• macOS ‘Mojave’ 10.14.6 and above.
• macOS ‘Catalina’ 10.15.3 and above. (1)
• macOS ‘Big Sur’ 11.1 and above. (1)
(1) Please note that there are currently some minor compatibility issues with macOS 11 and 10.15, which are detailed in the Hauptwerk release notice.
(Hauptwerk might possibly still work on OS X 10.12 or 10.11, but we’ll only provide support on them for issues that can be reproduced on one of our primary supported Mac platforms listed above, and that don’t appear to be related to the platform. Hauptwerk cannot be installed or used on OS X versions 10.10 or earlier
Hence macOS 10.13.6 is the earliest version it will definitely work on.