The current version of nagios-plugins-pgactivity in the yum repository "pgdg96" is 2.0. However, there was recently an update released which addresses support for PostgreSQL 9.6. The latest version is 2.2, see https://github.com/OPMDG/check_pgactivity/releases
Please update the package nagios-plugins-pgactivity to 2.2
Kind regards,
Pim Rupert