ac: 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/ac - from package: psacct

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

On Debian possible default full paths are:

/usr/bin/ac - from package: acct

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

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