sem: 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 Debian possible default full paths are:

/usr/bin/sem - from package: parallel

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