dcm2niigui: command not found

Have You tried accessing the binary with the full path?

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/dcm2niigui - from package: mricron

You may install the required package with command apt-get install {package}
Error in other languages:
dcm2niigui: bevel nie gevind nie
dcm2niigui: comandă negăsită
dcm2niigui: comando não encontrado
dcm2niigui: commande introuvable
dcm2niigui: command not found
dcm2niigui: käsku ei ole
dcm2niigui: không tìm thấy lệnh
dcm2niigui: komanda nerasta
dcm2niigui: Kommando nicht gefunden.
dcm2niigui: kommandot finns inte
dcm2niigui: komut yok
dcm2niigui: nie znaleziono polecenia
dcm2niigui: níor aimsíodh an t-ordú
dcm2niigui: no se encontró la orden
dcm2niigui: no s'ha trobat l'ordre
dcm2niigui: opdracht niet gevonden
dcm2niigui: parancs nem található
dcm2niigui: perintah tidak ditemukan
dcm2niigui: príkaz nenájdený
dcm2niigui: příkaz nenalezen
dcm2niigui: команда не найдена
dcm2niigui: командата не е открита
dcm2niigui: 命令找不到
dcm2niigui: コマンドが見つかりません