nbackup: 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/nbackup - from package: firebird

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

On Debian possible default full paths are:

/usr/bin/nbackup - from package: firebird2.5-classic-common
/usr/bin/nbackup - from package: firebird2.5-super

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

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