If the problem that the SD card is not readable on Mac doesn't lie in either the SD card or the MacBook(hardware-related issue), then it could be the problem with the macOS Big Sur. If you have inserted the SD card into Mac's and PC's slots directly or with the same card reader, and working cable, the connection issue is also eliminated.
I noticed some other Mac users have encountered the same issue with macOS Big Sur, it seems that it is a software glitch. And there are some working methods to make the SD card readable again:
Method 1. Reset SMC. For T2-secured Intel Mac: shut down the Mac, press and hold the right Shift key, the left Option key, and the left Control key for 7 seconds, then press and hold the power button as well, keep holding all four keys for another 7 seconds and release, then wait a few seconds and turn on your Mac.
For Intel Mac without T2 and Apple silicon Mac: power off the Mac, press and hold the left Shift-Control-option keys as well the power button for 10 seconds, release the keys, and turn on your Mac.
Method 2. Check Finder's preferences settings. Perhaps, it is the Finder's incorrect settings that cause the SD card not showing up/reading/recognizing on Mac. If so, you can go head to Finder and click Finder next to the Apple logo > Preferences, then check the General and Sidebar tabs, to tick the box of the external disks.
Method 3. Rename and reconnect the SD card. Some users found this method helpful. Go to Disk Utility and locate the SD card in the left sidebar, perhaps it is "untitled," right-click it and choose "Rename" your SD card, then right-click it again to choose "Eject" it, then reconnect the SD card to Mac.
Method 4. Update macOS. If this is a bug with macOS Big Sur, you can check whether there is an available update, maybe the newer macOS version can fix the SD card not reading issue. Go ahead to the Apple menu > System Preferences > Software Update.