tau_reduce: 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/tau_reduce - from package: tau
/usr/lib/tau/x86_64/bin/tau_reduce - from package: tau

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

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