qdoc: 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/qdoc - from package: qt5-qdoc
/usr/lib64/qt5/bin/qdoc - from package: qt5-qdoc

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

On Debian possible default full paths are:

/usr/bin/qdoc - from package: qtchooser
/usr/lib/x86_64-linux-gnu/qt5/bin/qdoc - from package: qtbase5-dev-tools

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

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