Apologies, I just re-read your post. Plexamp isn’t currently distributed for Linux on ARM (like Raspberry Pi), with the exception of Plexamp Headless. The flatpak is for x86_64 systems only, unfortunately.
You could use the headless install, and access it via the browser UI, but this would be a bit more involved to set up than the flatpak installer. See this thread for details:
