The 2nd Quadrant RHEL6 repo contains an RPM for barman 2.5 that uses python2.6.

Your barman 2.5 RPM for RHEL6 in the 9.4 repo <https://download.postgresql.org/pub/repos/yum/9.4/redhat/rhel-6-x86_64/barman-2.5-1.rhel6.noarch.rpm> is built for python3.4 but there aren't any RPM available for the prerequisite packages python-argh and python-argcomplete for python3.4. 

BTW, I tried to create a ticket and received an HTTP error 422 when trying to authenticate.

Thanks,
Matthew Musgrove