calcurse: 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 CentOS possible default full paths are:

/usr/bin/calcurse - from package: calcurse

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/calcurse - from package: calcurse

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

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