sftp-admin: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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-admin - from package: mysecureshell

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