imstack: 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/imstack - from package: wcstools

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

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