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

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

Error in other languages:
assistant-qt5: bevel nie gevind nie
assistant-qt5: comandă negăsită
assistant-qt5: comando não encontrado
assistant-qt5: commande introuvable
assistant-qt5: command not found
assistant-qt5: käsku ei ole
assistant-qt5: không tìm thấy lệnh
assistant-qt5: komanda nerasta
assistant-qt5: Kommando nicht gefunden.
assistant-qt5: kommandot finns inte
assistant-qt5: komut yok
assistant-qt5: nie znaleziono polecenia
assistant-qt5: níor aimsíodh an t-ordú
assistant-qt5: no se encontró la orden
assistant-qt5: no s'ha trobat l'ordre
assistant-qt5: opdracht niet gevonden
assistant-qt5: parancs nem található
assistant-qt5: perintah tidak ditemukan
assistant-qt5: príkaz nenájdený
assistant-qt5: příkaz nenalezen
assistant-qt5: команда не найдена
assistant-qt5: командата не е открита
assistant-qt5: 命令找不到
assistant-qt5: コマンドが見つかりません