To reset Premiere pro preferences, follow this step:
Press Alt (Windows) or Option (Mac OS) while launching Premiere Pro. You can release the Alt key or Option key when the splash screen appears. To reset preferences and plug-in cache at the same time, hold down Shift-Alt (Windows) or Shift-Option (Mac OS) while the application is starting. Release the Shift-Alt keys or Shift-Option keys when the splash screen appears.
Go to File > Project settings > General > Video Rendering & PlayBack > Change to mercury playback engine Software only.
Try updating the Premiere Pro 2021 version maybe the 2020 version has issues with macOS Big Sur 11.3. I am currently using Premiere Pro 2021 ver. 15.2 on macOS Big Sur 11.3 and it's working perfectly fine for me.
Vishu Aggarwal Adobe Certified Instructor, Professional and Expert