With a properly formatted APFS drive, you should see only a single Macintosh HD - Data volume. Although I can't tell from the image you provided, it appears that you have more than one of these, and one is grayed out. Is that correct? If so, this can happen if you chose a volume, and not a container, when you reinstalled macOS.
FYI ... macBig Sur will format the system drive for APFS. It then creates a single APFS container with two volumes. By default, those volumes are: Macintosh HD & Macintosh HD - Data.
If that grayed out volume is the additional HD - Data volume, then it can be deleted using the Disk Utility.
For reference, here is a screenshot from the Disk Utility on my Mac:
Be sure to change the view, in the Disk Utility, to "Show All Devices."