Isn't Rockbox Utility still a 32bit-app (as of v. 1.4.0) ? On Big Sur, at least, I'm unable to launch it for that reason. Catalina also supports 64bit apps only, as far as I'm aware.
While older versions of MacOS should be able to run the app, in my experience you need to launch Contents/MacOS/RockboxUtility inside the app bundle using sudo from the Terminal (i.e. with elevated rights), as Rockbox Utility will otherwise not work as expected.