For a long time, when running Disk Utility>First Aid on the boot/user volumes MacOS would complain that it couldn’t do everything it needed to. As a result I got into the habit of booting from MacOS Recovery and running DU>FA from there, then rebooting normally. Note that sometimes I have needed to run FA on the container while in Recovery, not just the volumes. This might address your error message. See my earlier discussion on how to do this.
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