Download Etrecheck, it is a diagnostic tool that often reveals what the cause of the problem is. Run Etrecheck and post the output here using the Additional text button below.
Also, you may have malware downloaded unintentionally. Etrecheck often shows this if it is present. You can also download and run the free version of Malwarebytes to remove any malware.
Do you have any "cleaning" programs or anti-virus or security suites installed? These often interfere with the MacOS normal processes. Etrecheck should also show those but if you have any such tools installed, best to completely uninstall them using the vendor's uninstallers.
I would do the above to get to and fix the root cause. However you can also boot into Safe Mode and try the update from there, that often bypasses installed items and MIGHT work.
Start up your Mac in safe mode - Apple Support