It has been reported that on macOS Big Sur with ESET Endpoint Antivirus for macOS installed, OS updates using Software Update will stop halfway through.
Reference：[NEWS7604] ESET support of macOS Big Sur (11) (eset.com)
For the latest version, 6.10.460.1, try booting in Start up your Mac in safe mode and then try the update.
19th March 2021
Information Processing Office.