It’s linked to RHEL 7, but base is RHEL 6 here.
Amazon Linux AMI 2015.03# yum install postgresql94-server postgresql94
Loaded plugins: priorities, update-motd, upgrade-helper
979 packages excluded due to repository priority protections
Resolving Dependencies
--> Running transaction check
---> Package postgresql94.x86_64 0:9.4.1-1PGDG.rhel7 will be installed
--> Processing Dependency: postgresql94-libs = 9.4.1-1PGDG.rhel7 for package: postgresql94-9.4.1-1PGDG.rhel7.x86_64
--> Processing Dependency: libpq.so.5()(64bit) for package: postgresql94-9.4.1-1PGDG.rhel7.x86_64
---> Package postgresql94-server.x86_64 0:9.4.1-1PGDG.rhel7 will be installed
--> Processing Dependency: systemd-units for package: postgresql94-server-9.4.1-1PGDG.rhel7.x86_64
--> Processing Dependency: systemd-units for package: postgresql94-server-9.4.1-1PGDG.rhel7.x86_64
--> Running transaction check
---> Package postgresql94-libs.x86_64 0:9.4.1-1PGDG.rhel7 will be installed
---> Package postgresql94-server.x86_64 0:9.4.1-1PGDG.rhel7 will be installed
--> Processing Dependency: systemd-units for package: postgresql94-server-9.4.1-1PGDG.rhel7.x86_64
--> Processing Dependency: systemd-units for package: postgresql94-server-9.4.1-1PGDG.rhel7.x86_64
--> Finished Dependency Resolution
Error: Package: postgresql94-server-9.4.1-1PGDG.rhel7.x86_64 (pgdg94)
Requires: systemd-units
You could try using --skip-broken to work around the problem
You could try running: rpm -Va --nofiles --nodigest