nifti1_test: 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/nifti1_test - from package: nifti-bin

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

Error in other languages:
nifti1_test: bevel nie gevind nie
nifti1_test: comandă negăsită
nifti1_test: comando não encontrado
nifti1_test: commande introuvable
nifti1_test: command not found
nifti1_test: käsku ei ole
nifti1_test: không tìm thấy lệnh
nifti1_test: komanda nerasta
nifti1_test: Kommando nicht gefunden.
nifti1_test: kommandot finns inte
nifti1_test: komut yok
nifti1_test: nie znaleziono polecenia
nifti1_test: níor aimsíodh an t-ordú
nifti1_test: no se encontró la orden
nifti1_test: no s'ha trobat l'ordre
nifti1_test: opdracht niet gevonden
nifti1_test: parancs nem található
nifti1_test: perintah tidak ditemukan
nifti1_test: príkaz nenájdený
nifti1_test: příkaz nenalezen
nifti1_test: команда не найдена
nifti1_test: командата не е открита
nifti1_test: 命令找不到
nifti1_test: コマンドが見つかりません