dropbearkey: 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/dropbearkey - from package: dropbear

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

On Debian possible default full paths are:

/usr/bin/dropbearkey - from package: dropbear

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

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