security update 2020-003 can't be installed on this disk.

If you have Security Update 2020-006, then you already have Security Update 2020-003 installed. Those suggestions were for issues occurring before the security update was available and that update may have resolved those issues at that time. Either the bug has returned or you have another slightly different issue (meaning perhaps something else is causing a similar problem). If the same bug has returned, then you may need to contact the developers of Serato DJ Pro for assistance.

I don't know anything about that software. You may find you will get more assistance on these forums if you create a new thread titled something like "Serato DJ Pro Freezing Issues" and explaining in detail what happens and the exact error messages encountered (if any). Please provide the exact model of your Mac. Even better run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper. The report will contain valuable information for other contributors to assist you including the exact model of your Mac and other important details.

It also never hurts to run the Apple Diagnostics or to run Disk Utility First Aid on the drive. Even if First Aid reports everything as "Ok" make sure to click "Show Details" and look for any unfixed errors. If there are unfixed errors listed, then you will need to erase the whole physical drive and restore from a backup or clone.

When creating your new post please let them know the steps you have already tried to resolve the issue. If you've run the diagnostics and First Aid and checked for unfixed errors, then please mention that as well so contributors are aware as it will save time.

Make sure to have a good frequent & regular backups.

Did you download this update through the Software Update panel? Or did you manually download this security update from the Apple website using a web browser?

Have you tried rebooting the laptop and trying again? Or trying to install the update while booted into Safe Mode?

If you check Software Update within System Preferences does it show any updates are needed or pending?

You may need another earlier update installed before this particular update can be installed. It is best to just allow macOS to update itself instead of downloading individual updates. Now if an update had problems installing through the normal automatic macOS update process, then trying a manual download & install of that particular update is Ok. Obviously somehow your macOS installation doesn't meet the requirements of this particular update. Maybe the update has already been applied automatically through the normal macOS update process. It is unfortunate that macOS doesn't provide a more detailed response for why your system doesn't meet the requirements of the update.

I also have this problem, finally find the answer

thanks from MiisooDoll