From the docs:
The RabbitMQ server scripts are installed into /usr/local/sbin. This is not automatically added to your path, so you may wish to add
PATH=$PATH:/usr/local/sbin to your .bash_profile or .profile. The server can then be started with rabbitmq-server.All scripts run under your own user account. Sudo is not required.
You should be able to run /usr/local/sbin/rabbitmq-server or add it to your path to run it anywhere.
Your command failed because, by default, . is not on your $PATH. You went to the right directory (/usr/local/sbin) and wanted to run the rabbitmq-server that existed and had exec permissions, but by typing rabbitmq-server as a command Unix only searches for that command on your $PATH directories – which didn’t include /usr/local/sbin.
What you wanted to do can be achieved by typing ./rabbitmq-server – say, execute the rabbitmq-server program that is in the current directory. That’s analogous to running /usr/local/sbin/rabbitmq-server from everywhere – . represents your current directory, so it’s the same as /usr/local/sbin in that context.