I drive a Apple MacBook 2015 with the last version of Monterey (12.7) and I have a Roon account, paid for this month. My music (ca. 7 GB) AIFF is on my external drive-section connected by usb. I cannot hear my music and the computer doesn't show it.
Just another user, chiming in before the pros get on stage …
Since we‘re missing device setup screenshots, and to start with the basics, have you gone into Settings - Audio, looked for the device you want to output to and clicked Enable?
@Marin_Weigel makes a good point above. Have you added your local library via USB into Roon via Roon Settings>Storage?
Are you having issues outside of Roon? The first step would be make sure you can access your music folder from the external USB you have connected. Typically the drive will pop up on your desktop which will allow you access. Is that the case?