ssh-agent: 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-agent - 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-agent - from package: openssh-client

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

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