BackupPC_nightly: 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/share/BackupPC/bin/BackupPC_nightly - from package: BackupPC

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

On Debian possible default full paths are:

/usr/share/BackupPC/bin/BackupPC_nightly - from package: BackupPC

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

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