cs2cs: 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/cs2cs - from package: proj

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

On Debian possible default full paths are:

/usr/bin/cs2cs - from package: proj-bin

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

Error in other languages:
cs2cs: bevel nie gevind nie
cs2cs: comandă negăsită
cs2cs: comando não encontrado
cs2cs: commande introuvable
cs2cs: command not found
cs2cs: käsku ei ole
cs2cs: không tìm thấy lệnh
cs2cs: komanda nerasta
cs2cs: Kommando nicht gefunden.
cs2cs: kommandot finns inte
cs2cs: komut yok
cs2cs: nie znaleziono polecenia
cs2cs: níor aimsíodh an t-ordú
cs2cs: no se encontró la orden
cs2cs: no s'ha trobat l'ordre
cs2cs: opdracht niet gevonden
cs2cs: parancs nem található
cs2cs: perintah tidak ditemukan
cs2cs: príkaz nenájdený
cs2cs: příkaz nenalezen
cs2cs: команда не найдена
cs2cs: командата не е открита
cs2cs: 命令找不到
cs2cs: コマンドが見つかりません