look: 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/look - from package: util-linux-ng

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

On Debian possible default full paths are:

/usr/bin/look - from package: bsdmainutils
/usr/lib/plan9/bin/look - from package: 9base

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

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