ncdu: 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/ncdu - from package: ncdu

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

On Debian possible default full paths are:

/usr/bin/ncdu - from package: ncdu

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

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