tsort: 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 CentOS possible default full paths are:

/usr/bin/tsort - from package: coreutils

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/tsort - from package: coreutils

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