I have followed all the steps until I try to download Big Sur through Utilities, I get the message that ‘ Failed writing body’ and the download stop

This is likely because you disk format has been screwed by the reset procedure. I had exatly the same issue and you probably have a good chance to recover soon. Just go back to Disk Utility and check if your volume is named Untitled. If, as I suspect is not the case, format the sub-disk under the top master one. This should rename your disk to Untitled. (Try anything you can until the disc is renamed, you may see some errors and I do not remember exactly what I did)

Once the disk is renamed, please cut and paste each command, line by line to check that everything goes right.

PS: The list of commands were not functioning correctly because the /Volumes/Untitled wasn't existing, leading to a download in the wrong, small place, with a stop at 21%

Good luck :-)

This seems to be helping me too, I'm over 50% downloaded now. It failed twice before I tried what you mentioned. Thanks!

woups wrote:

format the sub-disk under the top master one.

That "sub-disk" is originally named "Data" or "Data Volume". Select it in Disk Utility, choose the erase option, then a dialog box will open offering you a chance to rename it. It needs to be renamed to "Untitled".

Have you checked to make sure that you have enough space on your hard drive to download the update?

It will error every time if the space is not there.

Upper left corner of your screen you'll see the apple logo, click on it.

Choose - About this Mac > Storage.

The OP is referring to https://support.apple.com/en-us/HT211983

In particular, the last section: "Or use Terminal to reinstall". The instructions are unclear or incorrect. The terminal text refers to "/Volumes/Untitled". "Untitled" should be "Data" or maybe "Macintosh HD". I don't know personally, I'm no good with Terminal. The text as is causes the download to a volume where there is only room for 21% of the file.

I don't know how to offer feedback on that support document, maybe you do?

Please read the following user tip and restate your question/problem in detail: 

Writing an Effective Apple Support Communities Question - Apple Community  

What Mac model and year do you have?

Have you tried downloading from the System/Software Update preference pane?

What do you mean by "through Utilities"?

I'm having this same issue. Just recently bought the 13" M1 version of the Macbook and wanted to factory reset. I followed an article and cleared the main drive, after that I couldn't get in to the Macbook. I tried using the reinstall utility but that failed with "can't personalize" so then I got to this article and this article fails because the download of the pkg stops at 21% as others are reporting. Also it might be worth to mention once I followed all the steps in this article the main HD is unmounted (i went back into the drive utility and saw that).

His message is clear enough, please do not post the scindé of useless comment if you have no idea about it. This just add stress to that guy that is confronted to bad apple software, probably because it is in a too early stage.

I’m having the same issue let me know if you found an answer