First problem! Always look for errors in log files!!
Quick search on the community for pymodbus throws up this answer
You need to restart the service once installed…
sudo systemctl restart emonhub.service
However, I think the RFM2Pi config might be wrong - can you post that part of the config as well please.
Do you have an emonBase card? if not comment out the RFM2Pi interfacer config.