Starting from version 5.1.5 bis, Parabuild is fully compatible with BricsCAD V21.
To use Parabuild in combination with this new version of BricsCAD, you first need to install BricsCAD V21 and also update Parabuild to version 5.1.5 bis or newer.
After that, you can switch between platforms using the Startup Settings tool. This tool can be found in the Parabuild group in the Windows Start Menu.