I have bumped into this error and of course RHEL won't let you see how to address this issues unless you pay, regardless here is how to reason in order to see where the problem is originating:

error message:"can not find RHNS CA file: /usr/share/rhn/RHN-ORG-TRUSTED-SSL-CERT"

The clue is in the error message rhn, there are rpm packages associated with rhn and you need to ride it.

1) Find out what rpm packages are involved with rhn

rpm -qa|grep rhn

rhn-client-tools-2.0.2-17.el7.noarch
rhnsd-5.0.13-7.el7.x86_64
yum-rhn-plugin-2.0.1-9.el7.noarch
rhnlib-2.5.65-4.el7.noarch
rhn-check-2.0.2-17.el7.noarch
rhn-setup-2.0.2-17.el7.noarch


rpm -e rhn-client-tools-2.0.2-17.el7.noarch rhnsd-5.0.13-7.el7.x86_64 yum-rhn-plugin-2.0.1-9.el7.noarch rhnlib-2.5.65-4.el7.noarch rhn-check-2.0.2-17.el7.noarch rhn-setup-2.0.2-17.el7.noarch --nodeps

--nodeps ignores the dependencies errors

When you are getting this error ?
Can you elaborate so that could get more information.