assistant: 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/assistant - from package: qt3-devel
/usr/lib/qt-3.3/bin/assistant - from package: qt3-devel
/usr/lib64/qt5/bin/assistant - from package: qt5-assistant
/usr/lib64/qt4/bin/assistant - from package: qt-x11
/usr/lib/qt4/bin/assistant - from package: qt-x11

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

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

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