sftp-who: 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/sftp-who - from package: mysecureshell

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

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