If the "offscreen" plugin is missing, you may need to rebuild Qt with the necessary platform plugins. First, ensure you have the Qt source code:
git clone https://code.qt.io/qt/qt5.git
git checkout <qt_version_tag> # Replace <qt_version_tag> with the desired version tag, e.g., 5.15.2
Then configure and build Qt with the necessary platform plugins:
./configure -prefix /path/to/qt -opensource -confirm-license -nomake examples -nomake tests
/path/to/qt with the desired installation path.
After rebuilding Qt, you can follow the previous steps to rebuild Bitcoin Core and run it. This should ensure that the required Qt platform plugins, including "offscreen," are available.
Remember to replace
<qt_version_tag> with the actual version tag of the Qt version you want to build.