tac: 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 CentOS possible default full paths are:

/usr/bin/tac - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/tac - from package: coreutils

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

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