Install libqscintilla2-qt5-designer by entering the following commands in the terminal:
sudo apt update sudo apt install libqscintilla2-qt5-designer
Qt5 Designer plugin for QScintilla 2
QScintilla is a text editor for Qt5 with features especially useful when writing and debugging source code. These include support for syntax styling, error indicators, code completion, call tips and margins. . This package contains a plugin to use the QScintilla 2 widget in the Qt5 UI Designer.