qalculate-gtk: 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/qalculate-gtk - from package: qalculate-gtk

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

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