bibtex8: 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/bibtex8 - from package: texlive

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

On Debian possible default full paths are:

/usr/bin/bibtex8 - from package: texlive-binaries

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

Error in other languages:
bibtex8: bevel nie gevind nie
bibtex8: comandă negăsită
bibtex8: comando não encontrado
bibtex8: commande introuvable
bibtex8: command not found
bibtex8: käsku ei ole
bibtex8: không tìm thấy lệnh
bibtex8: komanda nerasta
bibtex8: Kommando nicht gefunden.
bibtex8: kommandot finns inte
bibtex8: komut yok
bibtex8: nie znaleziono polecenia
bibtex8: níor aimsíodh an t-ordú
bibtex8: no se encontró la orden
bibtex8: no s'ha trobat l'ordre
bibtex8: opdracht niet gevonden
bibtex8: parancs nem található
bibtex8: perintah tidak ditemukan
bibtex8: príkaz nenájdený
bibtex8: příkaz nenalezen
bibtex8: команда не найдена
bibtex8: командата не е открита
bibtex8: 命令找不到
bibtex8: コマンドが見つかりません