dropbear: 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/sbin/dropbear - from package: dropbear

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

On Debian possible default full paths are:

/usr/sbin/dropbear - from package: dropbear

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

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