designer: 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/qt-3.3/bin/designer - from package: qt3-designer
/usr/lib64/qt5/bin/designer - from package: qt5-designer
/usr/lib/qt5/bin/designer - from package: qt5-designer
/usr/lib64/qt4/bin/designer - from package: qt-devel
/usr/lib/qt4/bin/designer - from package: qt-devel

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

On Debian possible default full paths are:

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

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

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