dcmmkdir: 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/dcmmkdir - from package: dcmtk

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

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