yubiserver: 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 - from package: yubiserver

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

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