zless: 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/zless - from package: gzip

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

On Debian possible default full paths are:

/bin/zless - from package: gzip

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

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