qhelpconverter: 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/lib64/qt5/bin/qhelpconverter - from package: qt5-qttools-devel
/usr/lib/qt5/bin/qhelpconverter - from package: qt5-qttools-devel
/usr/bin/qhelpconverter - from package: qt-devel
/usr/lib64/qt4/bin/qhelpconverter - from package: qt-devel
/usr/lib/qt4/bin/qhelpconverter - from package: qt-devel

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

On Debian possible default full paths are:

/usr/bin/qhelpconverter - from package: qtchooser
/usr/lib/x86_64-linux-gnu/qt4/bin/qhelpconverter - from package: qt4-dev-tools
/usr/lib/x86_64-linux-gnu/qt5/bin/qhelpconverter - from package: qttools5-dev-tools

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

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