ykval-synchronize: 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/sbin/ykval-synchronize - from package: yubikey-val

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

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