ln: 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:

/bin/ln - from package: coreutils

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

On Debian possible default full paths are:

/bin/ln - from package: coreutils
/usr/lib/klibc/bin/ln - from package: klibc-utils

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

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