I've heard from a number of people using MacOS 11, also known as Big Sur, having issues running BikeCAD Pro.
To investigate the matter, I've purchased a Mac Mini with Apple's M1 chip. This one is running Mac OS Big Sur version 11.2.3.
I am relieved to report that BikeCAD Pro does actually run quite well on Big Sur but admittedly there is one thing you'll have to be careful to avoid doing as it is possible to cause the program to crash.
As you can see, after launching the program, there's no problem bringing up dialog boxes and for example changing dropouts, modifying the colour, adding some dimensions, stretching out the reach changing the seat angle.
Where we can run into trouble is if we click on this green button to maximize the main BikeCAD window. Doing this alone is not a problem. However, the instant you click an icon to open another dialog box, the interface will become unusable. You'll have to hover over the dock at the bottom of the screen to bring up the BikeCAD icon, right click on the icon and choose Force Quit.
I regret that at this time I do not have a fix for this. However, now that I have a Mac Mini, I'll do my best to resolve the issue if Apple doesn't do it first.
If a previous session of BikeCAD is causing the program to launch with the main window maximized, you can restore the default window sizes by searching for the BikeCAD_16.0_configuration folder and deleting the file dialogs1.xml. The only purpose of this file is to remember the size and position of dialog boxes and the main BikeCAD window.
Be aware that while it's dangerous to maximize the main BikeCAD window on Mac OS Big Sur, it's not a problem to drag the corner of the window until it fills the screen.