Unable to open Roon on 2012 MacMini due to outdated OS, preventing backup for Nucleus transfer (ref#DGY2XG)

You can temporarily reinstall version 2.64, which works with your outdated macOS.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):