Edit the 1.4.2. Using Red Hat Developer Tools section in this guide https://access.redhat.com/documentation/en-us/red_hat_developer_toolset/9/html-single/user_guide/index#sect-Red_Hat_Developer_Toolset-Install

In step 1 change subscription-manager repos --enable rhel-7-variant-devtools-rpms to subscription-manager repos --enable rhel-7-variant-rhscl-rpms and delete step 2