First of all, it seems that Soundspectrum G-Force hasn't been updated in quite some time. You might want to contact them for support as their software is still trying to plugin to iTunes which was discontinued by Apple in favor of the Music App. It is also an Intel only app that would require Rosetta 2 for Apple Silicon support.
Vendors need to continue to keep up with Apple, Microsoft, Google. Sometimes a vendor will simply stop updating their software and that's a bummer but it happens all the time. They have other products that are updated, perhaps Soundspectrum wasn't earning enough sales.
You might have some luck with Rogue Amoeba software, Audio Hijack and / or SoundSource. These tools appear to allow you to virtually control audio input and output so perhaps you could capture the audio and redirect it to Soundspectrum G-Force eliminating the need for iTunes.
There's multiple online services to create music visualization videos as well as Magic Music Visuals which appears to be up-to-date with support for newer macOS versions. Costs only a bit more than Soundspectrum G-Force.