This seems to be the main hint: “Unrecognized option: processManagement.authorization”. It’s just my assumption, but maybe your conf file wasn’t saved. See if you’ve saved the /etc/mongod.conf file after adding the security option in step 6.

#security
security:
  authorization: "enabled"

In nano, hit Write Out (Ctrl-O), then Enter to save, then Exit (Ctrl-X)