Norton AV and VPN extensions prevent OS update.

First, as Bob has already recommended uninstall Norton. It's junkware.

Now try to complete the update? If you're going thru the System/Software Update preference pane try this instead: download and apply the macOS Catalina 10.15.6 Combo Update.  

If that fails to fix it boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and run the combo updater from there. 

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

Uninstall Norton. It uses system resources while providing no benefit. It may cause issues.

macOS - Security - Apple

Protect your Mac from malware - Apple Support

Set up your Mac to be secure - Apple Support

Effective defenses against malware and ot… - Apple Community

Chiquitos Said:

“I installed norton antivirus and vpn on my macbook pro. Now i got a message that says that some extensions may prevent updating macOS. what should I do? Uninstall norton? and vpn? Is there a way to solve this issue?”

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Keep Security Software off of your Mac.

Uninstall Norton, using the Uninstaller. Get download the Uninstaller directly from Norton.

You Should Use Malwarebytes for Mac:

Malwarebytes searches for malware (short for malicious software) and spyware. Malware makes your Mac behave in a mislead manor. 

Malwarebytes is created by longtime users of these forums, making it the only reliable Security Software for a Mac. Remove what is found from the quarantine.

Downloads:

1. Malwarebytes Anti-Malware for Mac
2. Malwarebytes Uninstaller

What fixed it for you: removing Norton or the Safe Mode boot?