Great! I'm not 100% sure what's happened, but I think it's that the compiler tried to use the shared library since it was available, but for that to work you need to have /PATH/TO/itensor/lib/ in your LD_LIBRARY_PATH environment variable. So it's probably that you don't have it in your LD_LIBRARY_PATH.
If you're motivated to get to the bottom of this, could you update your environment variable, check that it's updated, then recompile ITensor with ITENSOR_MAKE_DYLIB=1 again, and finally check that it all works (hopefully)?
Personally I find the static library option good enough, but the dynamic library option is nice for certain people's setups or if they care about binary size, etc. We didn't used to offer it but a few users asked us pretty strongly for it.