According to a report at Mr. Macintosh, the 11.2 update of macOS Big Sur doesn’t check to make sure enough free disk space is available. The upgrade will start, then run out of space and fail.
Mr. Macintosh says that, if FV2 encryption is enabled, you’ll be locked out of your data. Per Apple’s notes, upgrading to macOS Big Sur for the first time requires at least 35.5 GB of available storage. However even if your Mac does not have 35.5 GB of storage available, macOS will try to install the Big Sur update, and that’s when the issue arises.
9to5Mac notes that if you have a backup of your data, you can simply erase the entire disk and reinstall macOS. However, recovering data without a backup can be very difficult.
9to5Mac also points out that with FileVault enabled, you have to connect your Mac to another Mac via Target Disk Mode in order to recover your files. If FileVault wasn’t enabled on your Mac, you can try to delete some files using the Terminal app in macOS Recovery, which will allow macOS to complete the update process successfully.