rbssh: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/rbssh - from package: ReviewBoard

You may install the required package with command yum install {package}

Error in other languages:
rbssh: bevel nie gevind nie
rbssh: comandă negăsită
rbssh: comando não encontrado
rbssh: commande introuvable
rbssh: command not found
rbssh: käsku ei ole
rbssh: không tìm thấy lệnh
rbssh: komanda nerasta
rbssh: Kommando nicht gefunden.
rbssh: kommandot finns inte
rbssh: komut yok
rbssh: nie znaleziono polecenia
rbssh: níor aimsíodh an t-ordú
rbssh: no se encontró la orden
rbssh: no s'ha trobat l'ordre
rbssh: opdracht niet gevonden
rbssh: parancs nem található
rbssh: perintah tidak ditemukan
rbssh: príkaz nenájdený
rbssh: příkaz nenalezen
rbssh: команда не найдена
rbssh: командата не е открита
rbssh: 命令找不到
rbssh: コマンドが見つかりません