Failed to mount volume for APFS snapshot purging

I had already erased my volume before installing Mojave and then Catalina wouldn't install and gave me this same error.

For me, I noticed a greyed out volume, "Untitled" in disk utilities.

In Terminal, type

diskutil list

Which gave:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk1         2.0 TB     disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +2.0 TB     disk1
                                 Physical Store disk0s2
   1:                APFS Volume Macintosh SSD           1.5 TB     disk1s1
   2:                APFS Volume Preboot                 26.5 MB    disk1s2
   3:                APFS Volume Recovery                507.4 MB   disk1s3
   4:                APFS Volume VM                      2.1 GB     disk1s4
   5:                APFS Volume Untitled                20.5 KB    disk1s5

Notice the last volume is that same "Untitled"?

So, I thought, why not remove this unused volume, so from online I found

diskutil apfs deleteVolume disk1s5

(make sure to to change the disk identifier)

Then I tried to install again and it worked this time and I am writing this note from Catalina, without losing anything!

Hope this helps someone else.

-Drew

Hello  Josh_FlashFlood,

Thank you for using Apple Support Communities! I understand from the information you provided that you're receiving an error when trying to update your Mac; I'd like to help.

When you installed Mojave it should have automatically formatted the drive to APFS, this may have not happened if a third party tool was used to install Mojave. In order to resolve this issue now and allow you to update, you will need to have a recent back of your content, which you mentioend you have, and then perform an erase and install of macOS Mojave. Once Mojave has been reinstalled to an APFS file system, you can then install Catalina. Here's a couple of resources to help:

How to erase a disk for Mac

remember when formatting the drive, you will need to select APFS as the format type

How to reinstall macOS from macOS Recovery

Restore your Mac from a backup

Hope this helps.

Best Regards.