Salomon,
As the first 4 lines of the warning say, you should probably not be using mpi-conduit.
I encourage you to try use of psm-conduit, but you should probably avoid ibv-conduit (IB Verbs emulation on Intel Omni-Path is pretty slow in my experience). However, the job launch problem (the rest of the warning you report) is mostly independent of the choice of network (since they all use mpirun as the default job launcher).
I would have suspected that in the slurm job you describe, mpirun would have been able to honor the 2node X 24process request. Since it is not, I think it may be necessary to provide the information explicitly. So, I would suggest "upcrun -N2 -n48 ..."
Even when you add the "-N2", I suspect you may encountering a bug in the integration between GASNet and Open MPI when the latter is used as a job launcher. This was reported previously at
https://upc-bugs.lbl.gov/bugzilla/show_bug.cgi?id=3803, and I believe the second attachment to that bug report should be sufficient to get "-N2" working correctly for you.
If you use the default mpirun to launch psm-conduit jobs, you will need to set "OMPI_MCA_btl=^psm2" in your environment to prevent a conflict (GASNet and MPI cannot both use psm/psm2 as their transport library).
If after following the steps above, job launch works as intended, then you can stop reading here.
Otherwise, we can pursue PMI (slurm) support.
Regarding the information you mention from the HipMer mailing list:
Berkeley UPC (and GASNet which is the network library below it) does not, in general, support launching mpi-conduit jobs using anything other than mpirun. If the Open MPI build was properly integrated with slurm, then I don't think you would have experienced any problem (it would have detected the 2node X 24core jobs geometry). So PMI/slurm support become relevant only if BOTH: you switch to psm-conduit (which I do recommend) AND cannot launch correctly following the advice I gave above.
It is possible that PMI/slurm support was detected at configure time. Since the steps to configure that are non-trivial, I suggest you first look for
checking for PMI spawner support... yes
in the output of the configure step. If it is present then we can proceed with setting up upcrun.conf.
If it is not present, then we can pursue locating your pmi.h and libpmi.
-Paul