Uppercase and lowercase letters are different in Unix/Linux systems, unlike Windows systems. Please, use:
Be sure that the soft is installed on system.
Beyond what Alexandre mentioned (using the correct case -- redhat-support-tool), it's possible that your $PATH variable doesn't include /usr/bin; you could determine that by trying the full path of the tool: /usr/bin/redhat-support-tool. If that still reports "command not found", then you'll want to install the corresponding package, with: yum install redhat-support-tool
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