Do you mean that you run Disk Utility First Aid to check the Macintosh HD - Data volume but get the failure error? Since the Macintosh HD - Data volume is part of the startup disk, I suggest you run First Aid to check it in macOS Recovery Mode.
When First Aid verifies and repairs a disk or volume, it will lock it first. However, if you run First Aid in normal mode, some programs including those that run in the background will keep writing or reading to the Macintosh HD - Data volume. Then, the volume cannot be locked for First Aid checking and repair, and Disk Utility First Aid keeps coming up with the same error.
You can enter your Mac to Recovery Mode and then run Disk Utility First Aid to check it there. If First Aid still sends out the same error, try erasing your Mac startup disk and then reinstall macOS.
