nib-ls: 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 Debian possible default full paths are:

/usr/bin/nib-ls - from package: python-nibabel

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

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