sss_useradd: 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/sbin/sss_useradd - 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_useradd - from package: sssd-tools

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