tac_plus: 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/sbin/tac_plus - from package: tacacs+

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

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