sss_usermod: 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_usermod - 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_usermod - from package: sssd-tools

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

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