In Application Platform Installation Methods in the chapter 2.5. Managing JBoss EAP installation channels in Red Hat JBoss Enterprise command are not proper formatted. 

jboss-eap-installation-manager channel remove --channel-name <CHANNEL_NAME> --dir eap-8

it should be 

./jboss-eap-installation-manager.sh channel remove --channel-name <CHANNEL_NAME> --dir eap-8 

could you please check it?
see the documentation: https://access.redhat.com/documentation/en-us/red_hat_jboss_enterprise_application_platform/8.0/html/red_hat_jboss_enterprise_application_platform_installation_methods/assembly_installing-jboss-eap-8-using-the-cli-installer-method_default#proc_managing-channels-your-installation-is-subscribed-to_default