Roon server not loading due to Mac OS limitations on Mac Mini (ref#4S9K7D)

You can temporarily reinstall version 2.64, which works with your outdated macOS. The download link is in the release notes:

Going forward, you need macOS 12 (Monterey), but most Intel Macs that don’t get the update from Apple can easily be updated with Open Core Legacy Patcher (OCLP):