Hello @Eric!
Unfortunately, You seem to have become a “victim” of the new Roon requirements for MacOS. What follows is a quotation form Roons release note of the new update.
“One important note: this release requires macOS 12 or later on Mac, and on Linux requires glibc 2.27 or later plus OpenSSL 1.1.1 or later. If you missed our earlier announcement and want to check whether your system is affected, more details are available in this post.
For Mac users who are not yet able to update to macOS 12 or later, we’re also making a pre-update installer available as a temporary option. This version will not receive future updates or security fixes, and may become incompatible with current Roon Remote or ARC versions over time. You can find the installer here.”
You can find the full release note here:
https://community.roonlabs.com/t/roon-2-65-and-arc-1-77-are-live/318732
I am afraid that in the long run You will have to upddate Your hardware. The cheapest solution now might be too look for a second hand Mac mini with the new Apple chip.
I am sorry for not being able to give You better news.
Stefan