ssh: 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/ssh - from package: openssh-clients

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

On Debian possible default full paths are:

/usr/bin/ssh - from package: openssh-client

You may install the required package with command apt-get install {package}

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