pynifti_pst: 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/pynifti_pst - from package: python-nifti

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

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