yubiserver-admin: 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/yubiserver-admin - from package: yubiserver

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

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