ARW files in photo 3.0

You 2011 iMac is running the latest system it can, High Sierra. So to get support for your camera's RAW files you'll need a Mac that can run Mojave or newer.

Or, you can convert those ARW files to dng files with Adobe's RAW Converter which earlier Photos should be able to handle.

Your camera model is not on the list for macOS 10.13 High Sierra: Digital camera RAW formats supported by iOS 11 and macOS High Sierra - Apple Support

But on the list for macOS 10.14 Mojave or newer: Digital camera RAW formats supported by iOS 15, iPadOS 15, and macOS Monterey - Apple Support

Have you been able to before?

RAW support goes by OS, yours might be too old for the RAWs for your camera. Can you import jpegs?

You're welcome.