sftp: 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/sftp - from package: openssh-clients

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

On Debian possible default full paths are:

/usr/bin/sftp - from package: openssh-client

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

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