pybtex-convert: 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/pybtex-convert - from package: pybtex

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

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