Yes, if you're running an ancient Mac, you're not going to be able to install the latest version of the converter in order to convert a ARW file from a current model Sony. All depends on which camera was used.
Time to upgrade your operating system or your Mac or ask the client to provide you with a DNG file.
If you upload your ARW file here, someone should be able to convert it for you.
A shame. My Mac does everything it needs to do. A waste to get rid of it. It has served me well. Just a tiny little software glitch has thrown a wrench into things! Client has sent me a different file format, so good for now.