gnucap-modelgen: command not found

Have You tried accessing the binary with the full path?

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/gnucap-modelgen - from package: gnucap

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

On Debian possible default full paths are:

/usr/bin/gnucap-modelgen - from package: gnucap

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