gtk-builder-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 CentOS possible default full paths are:

/usr/bin/gtk-builder-convert - from package: gtk2-devel

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

On Debian possible default full paths are:

/usr/bin/gtk-builder-convert - from package: libgtk2.0-dev

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

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