selinux-activate: 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

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/sbin/selinux-activate - from package: selinux-basics

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