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Two possible solutions to try;
1 - Restart in Safe Mode and attempting the Software Update again
2 - Restart in Recovery Mode Command + r immediately at restart and choose Reinstall macOS. This may sound off and it is off but the Software Update Applet and Apple Servers have been experiencing this issues for some time.
The 2 - suggestion will bring down the latest version of Big Sur 11.6.4 and commence the installation without touching an Personal Files, Applications, Settings or Preferences.
Suggest doing A Time Machine Backup before attempting either of the two above.