voms-proxy-init: 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/voms-proxy-init - from package: voms-clients-cpp

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

On Debian possible default full paths are:

/usr/bin/voms-proxy-init - from package: voms-clients

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

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