qtdiag: 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/qtdiag - from package: qt5-qttools-devel
/usr/lib64/qt5/bin/qtdiag - from package: qt5-qttools-devel
/usr/lib/qt5/bin/qtdiag - from package: qt5-qttools-devel

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

On Debian possible default full paths are:

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

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

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