gtkdoc-scanobj: 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/gtkdoc-scanobj - from package: gtk-doc

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

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