PROBLEM:
Client nodes have a down level Puppet yum repository, i.e. it should be 2018.1.9 in this case
[root@yum.repos.d]# cat pc_repo.repo
[pc_repo]
name=Puppet Labs puppet5 Repository
baseurl=https://puppet:8140/packages/2018.1.8/el-7-x86_64
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-puppetlabs
  file:///etc/pki/rpm-gpg/RPM-GPG-KEY-puppet
sslcacert=/etc/puppetlabs/puppet/ssl/certs/ca.pem
sslclientcert=/etc/puppetlabs/puppet/ssl/certs/clientnode.it.com.pem
sslclientkey=/etc/puppetlabs/puppet/ssl/private_keys/clientnode.it.com.pem
[root@ yum.repos.d]#

SOLUTION:
Create a group (for example 'Agent_Maintain')  Assign the puppet_agent class in the Configuration tab.
In the same tab, set the class' varaible:    "package_version": "5.5.6"
Pin the node.
Run the agent.
EOS