modhex: 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/modhex - from package: libyubikey

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

On Debian possible default full paths are:

/usr/bin/modhex - from package: libyubikey-dev

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

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