calcurse-upgrade: 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/calcurse-upgrade - from package: calcurse

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

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