Hi there ThongChi,
It appears that you're not able to add the battery percentage to the Menu Bar on your 2020 MacBook Air after updating it to macOS Big Sur 11.3. We know how important it is to know the battery percentage available, and we're glad to provide assistance.
How to use safe mode on your Mac - Apple Support -- Restart your Mac in safe mode and see if you can add the battery percentage there. Be aware that it takes a bit longer to restart in safe mode, as it stops certain software from loading, does a basic check of your startup disk, and deletes some system caches. Whether or not your able to add the battery percentage, you can restart normally again. If you were able to add the menu item, see if still there after restart.
New user or group setup - Apple Support -- If you weren't able to add the battery percentage, create a new admin account and see if it's showing there. If not, see if it can be added.
Please let us know if this works in safe mode or if it works in a new user account.
Take care.