s2tc_from_s3tc: 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/bin/s2tc_from_s3tc - from package: libtxc-dxtn-s2tc-bin

You may install the required package with command apt-get install {package}
Error in other languages:
s2tc_from_s3tc: bevel nie gevind nie
s2tc_from_s3tc: comandă negăsită
s2tc_from_s3tc: comando não encontrado
s2tc_from_s3tc: commande introuvable
s2tc_from_s3tc: command not found
s2tc_from_s3tc: käsku ei ole
s2tc_from_s3tc: không tìm thấy lệnh
s2tc_from_s3tc: komanda nerasta
s2tc_from_s3tc: Kommando nicht gefunden.
s2tc_from_s3tc: kommandot finns inte
s2tc_from_s3tc: komut yok
s2tc_from_s3tc: nie znaleziono polecenia
s2tc_from_s3tc: níor aimsíodh an t-ordú
s2tc_from_s3tc: no se encontró la orden
s2tc_from_s3tc: no s'ha trobat l'ordre
s2tc_from_s3tc: opdracht niet gevonden
s2tc_from_s3tc: parancs nem található
s2tc_from_s3tc: perintah tidak ditemukan
s2tc_from_s3tc: príkaz nenájdený
s2tc_from_s3tc: příkaz nenalezen
s2tc_from_s3tc: команда не найдена
s2tc_from_s3tc: командата не е открита
s2tc_from_s3tc: 命令找不到
s2tc_from_s3tc: コマンドが見つかりません