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 (19th March 2021), try booting in Start up your Mac in safe mode and then try the update.
We have confirmed that 6.10.910.0, the latest version as of May 25, 2021, can be updated from macOS Big Sur 11.3.1 to 11.4. We have not yet verified if this is possible on all devices.
19th March 2021
Information Processing Office.