ex4_acceptor: 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/ex4_acceptor - from package: ctn

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

Error in other languages:
ex4_acceptor: bevel nie gevind nie
ex4_acceptor: comandă negăsită
ex4_acceptor: comando não encontrado
ex4_acceptor: commande introuvable
ex4_acceptor: command not found
ex4_acceptor: käsku ei ole
ex4_acceptor: không tìm thấy lệnh
ex4_acceptor: komanda nerasta
ex4_acceptor: Kommando nicht gefunden.
ex4_acceptor: kommandot finns inte
ex4_acceptor: komut yok
ex4_acceptor: nie znaleziono polecenia
ex4_acceptor: níor aimsíodh an t-ordú
ex4_acceptor: no se encontró la orden
ex4_acceptor: no s'ha trobat l'ordre
ex4_acceptor: opdracht niet gevonden
ex4_acceptor: parancs nem található
ex4_acceptor: perintah tidak ditemukan
ex4_acceptor: príkaz nenájdený
ex4_acceptor: příkaz nenalezen
ex4_acceptor: команда не найдена
ex4_acceptor: командата не е открита
ex4_acceptor: 命令找不到
ex4_acceptor: コマンドが見つかりません