You should try to recover pass4SymmKey by using another one of your server in the cluster or reuse splunk.secretfrom another member of your cluster then take the encrypted pass4SymmKey from there to your new server.
Check your clustering configuration in server.conf on new Indexer which you are trying to add. The error says "Unauthorized" which indicates that credentials are not correct. We need a secret key "pass4SymmKey" in order to add a new Indexer to the existing cluster. Please make sure it is correct.