sss_groupmod: 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/sbin/sss_groupmod - from package: sssd-tools

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

On Debian possible default full paths are:

/usr/sbin/sss_groupmod - from package: sssd-tools

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

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