stk-demo: 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/stk-demo - from package: stk

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

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