gtkdoc-mkhtml: 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-mkhtml - from package: gtk-doc

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

On Debian possible default full paths are:

/usr/bin/gtkdoc-mkhtml - from package: gtk-doc-tools

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

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