Thanks BryanC.
I returned the jriver.list file back to bullseye, and run the install. Results below.
pi-2@raspberrypi:~ $ ./installJRMC --debug
Debug: Running: main --debug
Debug: Running: init
Starting installJRMC
Debug: Detected host platform: debian 12 arm64
Debug: Using host platform: debian 12
Debug: Running: parseInput
Debug: Debugging on
Debug: installJRMC version: 1.0-dev
Debug: Running: setMCVersion
Determining latest MC version
Debug: Running: installPackage --silent buildah
Debug: buildah already installed, skipping installation
Debug: buildah rm debian-working-container
b25a5d27c815be8db74417e397b681614de0e9f830f31695f58458155a4cb433
Using MC version 32.0.20-arm64 determined by containerized package manager
To override, use --mcversion
Debug: MVERSION: 32, MC_VERSION: 32.0.20-arm64, MC_PKG: mediacenter32, MC_RPM: /home/pi-2/output/RPMS/x86_64/mediacenter32-32.0.20-arm64.x86_64.rpm
Installing JRiver Media Center from remote repository
Debug: Running: installMCFromRepo
Debug: sudo rm -rf /etc/apt/sources.list.d/mediacenter*.list
Debug: Running: installPackage wget
Debug: wget already installed, skipping installation
Debug: sudo apt-get update -y -q0
Hit:1
http://deb.debian.org/debian bookworm InRelease
Get:2
http://deb.debian.org/debian-security bookworm-security InRelease [48.0 kB]
Get:3
http://deb.debian.org/debian bookworm-updates InRelease [55.4 kB]
Get:4
http://dist.jriver.com/latest/mediacenter bullseye InRelease [3,614 B]
Hit:5
http://archive.raspberrypi.com/debian bookworm InRelease
Get:6
http://dist.jriver.com/latest/mediacenter bullseye/main amd64 Packages [853 B]
Get:7
http://dist.jriver.com/latest/mediacenter bullseye/main armhf Packages [860 B]
Get:8
http://dist.jriver.com/latest/mediacenter bullseye/main arm64 Packages [831 B]
Get:9
http://dist.jriver.com/latest/mediacenter bullseye/main i386 Packages [871 B]
Fetched 110 kB in 3s (38.2 kB/s)
Reading package lists... Done
Debug: Running: installMesa
Debug: Running: installPackage --no-install-check --allow-downgrades --no-gpg-check mediacenter32
Debug: sudo apt-get -f install -y -q0 --allow-downgrades mediacenter32
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
mediacenter32 is already the newest version (32.0.20-arm64).
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
JRiver Media Center installed successfully from remote repository
Debug: Running: symlinkCerts
Debug: Running: migrateLibrary
Debug: Running: restoreLicense
Debug: Running: openFirewall jriver-mediacenter 52100-52200/tcp 1900/udp
Warning: Install firewall-cmd or ufw to open firewall ports
Debug: Running: disableCoW
pi-2@raspberrypi:~ $
MC32 appears as before in the drop down menu under Sounds & Video, but does not open.
If you don't see the error, I could try Bobs post from Jan 19, 2024 Adding the MediaCentre repo manually. In his post for adding the repo file list he does create for bookworm a jriver.list file ending with "bookworm main"