s2tc_compress: 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 Debian possible default full paths are:

/usr/bin/s2tc_compress - from package: libtxc-dxtn-s2tc-bin

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

Error in other languages:
s2tc_compress: bevel nie gevind nie
s2tc_compress: comandă negăsită
s2tc_compress: comando não encontrado
s2tc_compress: commande introuvable
s2tc_compress: command not found
s2tc_compress: käsku ei ole
s2tc_compress: không tìm thấy lệnh
s2tc_compress: komanda nerasta
s2tc_compress: Kommando nicht gefunden.
s2tc_compress: kommandot finns inte
s2tc_compress: komut yok
s2tc_compress: nie znaleziono polecenia
s2tc_compress: níor aimsíodh an t-ordú
s2tc_compress: no se encontró la orden
s2tc_compress: no s'ha trobat l'ordre
s2tc_compress: opdracht niet gevonden
s2tc_compress: parancs nem található
s2tc_compress: perintah tidak ditemukan
s2tc_compress: príkaz nenájdený
s2tc_compress: příkaz nenalezen
s2tc_compress: команда не найдена
s2tc_compress: командата не е открита
s2tc_compress: 命令找不到
s2tc_compress: コマンドが見つかりません