copac2xml: 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/copac2xml - from package: bibutils

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

On Debian possible default full paths are:

/usr/bin/copac2xml - from package: bibutils

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

Error in other languages:
copac2xml: bevel nie gevind nie
copac2xml: comandă negăsită
copac2xml: comando não encontrado
copac2xml: commande introuvable
copac2xml: command not found
copac2xml: käsku ei ole
copac2xml: không tìm thấy lệnh
copac2xml: komanda nerasta
copac2xml: Kommando nicht gefunden.
copac2xml: kommandot finns inte
copac2xml: komut yok
copac2xml: nie znaleziono polecenia
copac2xml: níor aimsíodh an t-ordú
copac2xml: no se encontró la orden
copac2xml: no s'ha trobat l'ordre
copac2xml: opdracht niet gevonden
copac2xml: parancs nem található
copac2xml: perintah tidak ditemukan
copac2xml: príkaz nenájdený
copac2xml: příkaz nenalezen
copac2xml: команда не найдена
copac2xml: командата не е открита
copac2xml: 命令找不到
copac2xml: コマンドが見つかりません