Can't partition disk for Boot Camp

Were you using Bootcamp Assistant to partition the drive when the error occurred?

What is the exact model of your Mac? You can get this information by clicking the Apple menu and selecting "About This Mac".

Do you have enough free space on the drive to allow it to be resized for Windows?

Launch the Terminal app and run the following command:

diskutil  list  internal

Post the output of this command here so we can see the current layout of the drive.

Try running the Apple Diagnostics to see if any hardware issues are detected.

Well Bootcamp Assistant never modified your drive layout which is good.

Apple says Bootcamp requires macOS to have at least 64GB of Free storage space available to use Windows 10....it seems Windows 10 itself requires at least 32GB of space....probably needs more to install OS updates and of course more space would be needed for any apps you wanted to installed into Windows....plus some extra free space for temp & cache files, plus any files you may need to save. Just make sure you keep at least 20GB+ of Free space available for macOS at all times....that 20GB can be used up fairly quickly by macOS depending on the workload so you may need to keep even more free space for macOS (you don't want to run out of free space on an APFS volume or you may get locked out & be unable to delete any files resulting in the need for a clean install of macOS).

Install Windows 10 on your Mac with Boot Camp Assistant - Apple Support

Now when you say you have "85GB available"....what exact value are you referencing? Look in Disk Utility and click on your "Data" volume. The only value of importance is the one for "Free" space which is the actual amount of storage space immediately available for use. This value should always be at least 20GB+ at all times. For a Windows Bootcamp installation, this value needs to be 64GB+. There is an "Available" value also listed in Disk Utility which is very misleading....I really wish Apple would remove it or change it to just "Purgeable". The "Available" value in Disk Utility represents the storage space that will be released as "Free" space at some unknown point in the future (It is actually comprised of "Purgeable" storage which will be released at some point and the current Free space).

If you have more than 64GB of Free space, then try running Disk Utility First Aid on the hidden Container. Within Disk Utility you may need to click "View" and select "Show All Devices" before the hidden Container appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok", click "Show Details" and scroll back through the report to see if any unfixed errors remain. If so, then you will need to run First Aid from Recovery Mode (Command + R), otherwise you will need to perform a clean install of macOS by first erasing the whole physical SSD (Intel Macs only) followed by reinstalling macOS & restoring from a backup.

The only reason I can think that the partitioning step failed is due to either a corrupt APFS file system, or there isn't enough free space available to do so.

DrYeet wrote:

So...

How much Free space is being reported by Disk Utility?

When running First Aid make sure to run First Aid on the hidden Container. Within Disk Utility you may need to click "View" and select "Show All Devices" before the hidden Container appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok", click "Show Details" and scroll back through the report to see if there ar any unfixed errors listed.

Also run First Aid on the physical drive which should be the top most item usually identified by the make & model of the drive...something like "Apple SSD S0256....".

You can try checking the health of the SSD by running DriveDx and posting the complete text report here using the "Additional Text" icon which looks like a piece of paper.

If the error you received did not mention any other details about why it could not partition the drive, then the only other thing I can suggest is to perform a clean install of macOS which involves erasing the whole physical before reinstalling macOS and trying Bootcamp Assistant again. Of course erasing the drive destroys all data on it, so make sure to have a good backup (you should always have a good backup anyway).

A quick search reveals this Apple article about what to do if BootCamp Assistant is unable to partition the drive. The only additional item this article mentions is to temporarily turn off Time Machine...it only mentions it for Mojave & High Sierra, but it couldn't hurt to try with Big Sur or Ventura.

If Boot Camp Assistant says that your disk could not be partitioned - Apple Support

I also found a reference to running a file system check using the command line using a special option while booted into Single User Mode, but I don't believe Single User Mode exists with a 2020 Mac with T2 chip. You should be able to do so while booted into Recovery Mode/Internet Recovery Mode, but I don't recall how to target the internal drive for this method and whether the volumes must first be unmounted. If you are familiar with the command line, then this may be an option assuming First Aid had "warnings" about over allocations. A clean install would do the same thing since the drive would be erased creating a clean new file system.

If you have Filevault enabled, perhaps you should disable Filevault. If you have a firmware password lock enabled, then try disabling it as well. These are not defaults so if they are enabled, then you should know how to disable them.

Unless another contributor has any other better ideas, this is all I can suggest to you. Unfortunately Apple is not very good at providing error messages with useful details. You can also try contact Apple support directly to see if they have any other advice.