It seems like a build or publish error has occurred with the RHEL6 repository for v4.2.9 RPMs. 4.2.9 is missing in el6 but present in el7 repos..

https://repo.mongodb.org/yum/redhat/6/mongodb-org/4.2/x86_64/RPMS/

This causes installation breakage:

You could try running: rpm -Va --nofiles --nodigest
{{ STDERR: Error: Package: mongodb-org-4.2.8-1.el6.x86_64 (mongodb-org-4.2)}}
{{ Requires: mongodb-org-server = 4.2.8}}
{{ Installed: mongodb-org-server-4.2.9-1.el6.x86_64 (@mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.9-1.el6}}
{{ Available: mongodb-org-server-4.2.0-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.0-1.el6}}
{{ Available: mongodb-org-server-4.2.1-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.1-1.el6}}
{{ Available: mongodb-org-server-4.2.2-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.2-1.el6}}
{{ Available: mongodb-org-server-4.2.3-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.3-1.el6}}
{{ Available: mongodb-org-server-4.2.5-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.5-1.el6}}
{{ Available: mongodb-org-server-4.2.6-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.6-1.el6}}
{{ Available: mongodb-org-server-4.2.7-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.7-1.el6}}
{{ Available: mongodb-org-server-4.2.8-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-server = 4.2.8-1.el6}}
{{ Error: Package: mongodb-org-4.2.8-1.el6.x86_64 (mongodb-org-4.2)}}
{{ Requires: mongodb-org-mongos = 4.2.8}}
{{ Installed: mongodb-org-mongos-4.2.9-1.el6.x86_64 (@mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.9-1.el6}}
{{ Available: mongodb-org-mongos-4.2.0-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.0-1.el6}}
{{ Available: mongodb-org-mongos-4.2.1-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.1-1.el6}}
{{ Available: mongodb-org-mongos-4.2.2-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.2-1.el6}}
{{ Available: mongodb-org-mongos-4.2.3-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.3-1.el6}}
{{ Available: mongodb-org-mongos-4.2.5-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.5-1.el6}}
{{ Available: mongodb-org-mongos-4.2.6-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.6-1.el6}}
{{ Available: mongodb-org-mongos-4.2.7-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.7-1.el6}}
{{ Available: mongodb-org-mongos-4.2.8-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-mongos = 4.2.8-1.el6}}
{{ Error: Package: mongodb-org-4.2.8-1.el6.x86_64 (mongodb-org-4.2)}}
{{ Requires: mongodb-org-shell = 4.2.8}}
{{ Installed: mongodb-org-shell-4.2.9-1.el6.x86_64 (@mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.9-1.el6}}
{{ Available: mongodb-org-shell-4.2.0-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.0-1.el6}}
{{ Available: mongodb-org-shell-4.2.1-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.1-1.el6}}
{{ Available: mongodb-org-shell-4.2.2-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.2-1.el6}}
{{ Available: mongodb-org-shell-4.2.3-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.3-1.el6}}
{{ Available: mongodb-org-shell-4.2.5-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.5-1.el6}}
{{ Available: mongodb-org-shell-4.2.6-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.6-1.el6}}
{{ Available: mongodb-org-shell-4.2.7-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.7-1.el6}}
{{ Available: mongodb-org-shell-4.2.8-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-shell = 4.2.8-1.el6}}
{{ Error: Package: mongodb-org-4.2.8-1.el6.x86_64 (mongodb-org-4.2)}}
{{ Requires: mongodb-org-tools = 4.2.8}}
{{ Installed: mongodb-org-tools-4.2.9-1.el6.x86_64 (@mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.9-1.el6}}
{{ Available: mongodb-org-tools-4.2.0-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.0-1.el6}}
{{ Available: mongodb-org-tools-4.2.1-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.1-1.el6}}
{{ Available: mongodb-org-tools-4.2.2-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.2-1.el6}}
{{ Available: mongodb-org-tools-4.2.3-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.3-1.el6}}
{{ Available: mongodb-org-tools-4.2.5-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.5-1.el6}}
{{ Available: mongodb-org-tools-4.2.6-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.6-1.el6}}
{{ Available: mongodb-org-tools-4.2.7-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.7-1.el6}}
{{ Available: mongodb-org-tools-4.2.8-1.el6.x86_64 (mongodb-org-4.2)}}
{{ mongodb-org-tools = 4.2.8-1.el6}}