g_polystat: 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/g_polystat - from package: gromacs

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

On Debian possible default full paths are:

/usr/bin/g_polystat - from package: gromacs

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