That makes sense, I didn’t think of that scenario.
I apologise, I got the Ubuntu versions mixed up. fenicsx is available from Ubuntu 20.04 (focal), it’s not available on 18.04 (bionic). FEniCS-X needs versions of libraries like PETSc that are more recent than the ones available on 18.04. Possibly this is related to the build problem you’re seeing, the available libraries might not be recent enough.