dcmodify: 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/dcmodify - from package: dcmtk

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

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