Wait, if you are using lxplus that’s not Ubuntu. You are on lxplus9, which uses AlmaLinux9. The ROOT releases on cvmfs older than 6.30 don’t support this platform yet. But what you can do is to source an LCG software stack that was built for alma 9 and includes ROOT, for example:
source /cvmfs/sft.cern.ch/lcg/views/LCG_105/x86_64-el9-gcc11-opt/setup.sh
The good thing about the LCG releases is that they ship the newest ROOT with all features enabled, so in this case a build of ROOT 6.30.02 that also includes TUnfold!
To check what ROOT version is contained in an LCG release, you can visit lcginfo.cern.ch.
Does connecting to lxplus and then sourcing the LCG release work for you?
Cheers,
Jonas