Time Machine backup error

Yep! Somewhere in the process you used, not only was there a second Macintosh HD - Data volume created, so was a Macintosh HD - Info volume ... and it's the second - Data volume that is confusing Time Machine. Not sure how you got the - Info volume created or what you are using it for.

Starting with macOS Mojave, Apple introduced the APFS file system. This system relies on containers and volumes. Think of containers as what was called partitions before. During the upgrade process to macOS Catalina, a single APFS container would be created on the Macintosh HD drive that would have, as a minimum, five volumes: Macintosh HD (which is read only and contains the basic operating system files), Macintosh HD - Data (which is read/write and contains some system files but primarily your apps and data), VM, Recovery (the "recovery" partition), and Preboot. With this default configuration, Time Machine shouldn't have any issues backing up.

Now that this default configuration was changed, you basically have two choices:

1. Boot up your Mac in Recovery Mode, completely erase the internal drive, and then, reinstall a clean copy of macOS Catalina, OR
2. Attempt to delete the extra - Data & - Info volumes using the Disk Utility. Which - Data volume you would delete would be the issue.

Regardless of which path you take, you will need to rely on a good backup to restore your apps and data. I would recommend the first option. If this seems a bit too overwhelming, you may want to schedule a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider to assist you with this.

Did you recently upgrade your iMac to macOS Catalina? If so, it appears that the upgrade process created two "Macintosh HD - Data" volumes. It should have only created one.

You can verify this by entering the following command in the Terminal app: diskutil list (If you need help interpreting the results, please post it so we can assist you.)

The results should look something like the following image:

I did read the error report but I don't know where those disks are located so I can rename one of them. That's what I'm trying to find out: how do I get to those disks that have the same name? THNX!

Ok, that was rkaufmann87 that asked you that question, not me. I'm sure you'll get a response.

I actually had to erase my computer and reload everything! That's when it created the 2nd disk. I don't know how to get to the "Terminal app" so not sure I can try your suggestion. THNX

The Terminal app is located in the /Applications/Utilities folder.

The error report is telling you what to do, did you read it?

You're very welcome!