macOS Catalina 10.15.5 won't update on my 13" MacBook Air 2015

Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and try to download and install the macOS Catalina 10.15.5 Combo Update  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. 

Having the exact same issue with my MBP 13" 2020 and seems unable to update no matter what I do, leaving it as it is for now, and having need to see the constant showing of update notification, which is frustrating.

Suggest going to https://support.apple.com/kb/DL2040?viewlocale=en_CA&locale=en_CA and download the Combo 10.15.5 update. Shutdown computer. Connect to power supply. Startup in Safe Mode ( Shift key at startup ) It will take longer to startup - this is normal. Once up, run the Combo Updater 10.15.5

Boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and try to install 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. 

I began a whole new discussion separate from this one for those of us with the mutual problem of update failure in spite of having more than sufficient space available... https://discussions.apple.com/thread/251578758

I had this problems for months with the same laptop, so that I couldn't install 10.15.5 and then I couldn't install 10.15.6.

I noticed there were several downloads in system files for both updates that weighted around 2 GB each and I could not erase, so it appeared the OS itself was downloading them (probably incomplete downloads) and failed several times to install.

These are the exact steps that worked for me:

• Release as much space as I could in my disk (around 25-30 GB).
• Restart on Safe Mode, download and install the 10.15.5 Combo Update from the website.
• Wait like 1 hour for it to finish (screen time said 20 min).
• Restart on Safe Mode, download and install the 10.15.6 Combo Update from the website.
• Wait like 1 hour for it to finish (screen time said 23 min).
• Restart on Safe Mode, and then restart normally again.
• After the automatic cache clean up from restarting on Safe Mode, I had like 40 GB of free space because of the original downloads I could not erase, were flushed by the system. This way I could restore my documents and keep using my laptop as usual.

I can't install the Catalina 10.5.5 update either. I have a 2017 MacBook Pro with 138GB of space and when I try to install 10.15.5, I receive the message that it can't be installed on this volume:

I followed Mr. Phillips' advice to shut down and startup in Safe Mode before running the Combo Updater 10.15.5. Before I did this, I downloaded the 'non-combo' update and received the same screen warning message. Neither update is showing up in my Software Update System Prefs. Am I receiving this screen message because I'm currently running the OS 10.15 (19A558d) Beta? From what I've read, after I unenrolled from the Beta Program, the next regular release would just replace the Beta version. Do I need to use Time Machine to reinstall the last regular OS version or can I just download that and install?

Thanks in advance!

I currently have over 385 GB of Flash storage available. I downloaded this 10.15.5 combo update to try in place of the original update that was downloaded through system update. I shut down and booted into Safe Mode as suggested. I ran the Combo Updater 10.15.5 then in Safe Mode and received this message...

... I am still looking for ideas or suggestions to be able to update from 10.15.4 to 10.15.5

That's strange. It doesn't show in my System/Software Update preference pane, the Apple downloads site or the App Store. Where are you getting it?

In any case I'll wait for the Combo updater to be released on the Downloads site.

I find the biggest culprit in robbing your Mac of storage is Podcasts. If you are subscribed to a few, they are downloaded to your Mac, whether you listen or not. All you need to do is to REMOVE ones that are old or that you have no interest in. This will cure a lot of problems. It did for me.

Since you are already on Catalina 10.15.xx, should be good for Security Updates for another 2 to 3 years. If you are asking if 2012 MacBook Air can upgrade beyond Catalina . . . Only Apple knows for sure.

Me, have Mac mini 2011 that can not go further than High Sierra 10.13.6, iMac Late 2012 on Catalina and do not expect to go any further. Also, 2018 Retina MacBook Air also on Catalina and expect to get one or two more upgrades 10.16 or 10.17 and end of the line.

Hope this answers the second question.

It makes total sense, thank you. Managed to update to Catalina 10.15.5 yayyy As for my MacBook Air 2012 I will wait and see. I also doubt it will be able to go beyond Catalina; however as long as Apple releases security only updates for it for another year or two I would be happy. My aim is to get 10 years out of it before I go ahead and buy a new MacBook Air. I think the new Macbok Air 2020 needs either one or two more refreshes before its really the best it can be ;)) Improved silent fans and a 1080HD FaceTime camera will make it a treasure haha

The solution I received from Apple Tech Support was to open the App Store app and search for 'Catalina.' The macOS Catalina link appeared and I clicked on 'Get.' The 8GB software update downloaded and was installed without any dialogue windows to click. Although the App Store link shows the 'Version History' only to 10.15.4, the latest update (10.15.5) was downloaded and installed.

Another solution is to pull down the 'Help' menu in the Finder and search for 'recovery.' Click on the match for 'Reinstall MacOS' and follow the instructions to update the software.

I hope this helps. Good luck!