Failed to personalise update : Sonoma 14.7 and Sequoia 15.0.1

Just to add to the good advise by @steve626

There are factors:

Some which are beyond your control and those with are totally under your control

Slow wifi connection

Software on the computer interfering with the download process including AntiVirus Software

Notation: Disabling the AV Software may or may not solve the issue.

Removal of the AV Software may increase the chances of the download to perform as expected.

Running behind a VPN which may increase Packet losses or corruption of the download 

If via wifi  & too many Other devices using the wifi at the same time reducing the Bandwidth to your computer. 

Best Results are via Ethernet Cable to Router

Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal 

Try Again using the Software Update Applet in System Preferences / Setting 

Worst case - Restart in Recovery Mode and choose Reinstall macOS. 

This in theory will pull the latest version of macOS this computer qualifies to run 

ybsid wrote:

My internet connection is about 100 Mbps and stable enough to stream 4K videos and online gaming. Ping is less than 100ms for remote servers.

I have tried turning off `Find my` on my iCloud as well as turning off `FileVault` as per other internet answers but still can't update.

One reason this can happen is a problem with the internet link. Can you connect directly to the router with an ethernet cable? The problem might not be your internet connection but it could be RF interference on the WiFi.

100 Mbps is not a very fast connection, how are you getting that, cable connection, DSL, other ...? And ping of 100 ms is quite poor, that indicates delays are occurring between your Mac and the internet provider. Could occur almost anywhere between. A good ping is 10 ms. Are you using a VPN, that can slow down everything and create delays and hitches in the internet link.

You could go to an Apple store and try the update there, they provide a free, stable high speed internet signal.

While I've not had this specific error message; I have encountered similar instances of a corporate network blocking access to portions of iCloud that have some impact on downloading macOS updates / upgrades. In one case, I had to use a mobile hotspot on my iPhone or an alternate WiFi network that wasn't being heavily inspected and filtered by corporate firewalls, etc. If you are running a corporate VPN you had better check with your IT department before you upgrade to macOS Sequoia, Sonoma should be fine but Sequoia has some VPN bugs.

If the Mac is portable try another Internet connection such as a public wifi access point that is fast. Apple store is a good option but also a public library, coffeeshop, etc. I don't recommend using a cellular hotspot if you can avoid it but if you have enough data and speed it will work.

Follow on

If an error occurred while updating or installing macOS - Apple Support