nl-class-add: 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/sbin/nl-class-add - from package: libnl3-cli

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

On Debian possible default full paths are:

/usr/sbin/nl-class-add - from package: libnl-utils

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

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