Postgres FATAL: could not open relation mapping file
I had the same error when I tried to reinstall PostgreSQL on RHEL6 using a Chef cookbook. In my case this seems to have been due to an incorrect manual uninstallation of PostgreSQL. I used this command to find the offending files that disrupted my installation:

find / -user postgres  


And then I deleted all the files I found as they were owned by postgres and I did not even have a postgres user. To be on the safe side I also rebooted the machine and after that it worked like a charm.