The Software Update Applet and Apple Delivery System has had issues since the release of Big Sur in November 2020.
Suggest Restarting the computer is Safe Mode ( SHIFT key immediately at restart. Does a Repair Disk, clears cache files and only loads Apple Software so loading is slow - Normal.
Ten run the Software Update in this mode.
If no success - restart in Normal Mode and they again. Still not success.
Restart in Recovery Mode Command + r immediately at restart and choose Reinstall macOS. Yes not a typo Reinstall macOS.
This should download the latest version 11.6.4 and commence the installations.
It should in theory not touch any personal files, preferences, setting or installed applications
Good Idea is to perform a Time Machine Backup Before attempting any of the above.