Most likely the file system is corrupt. You can try booting into Internet Recovery Mode and running Disk Utility First Aid on the hidden Container. Within Disk Utility you may need to click "View" and select "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Even if First Aid says everything is "Ok" click "Show Details" and scroll back through the report to see if any unfixed errors are listed. If there are any unfixed errors listed, then you will need to erase the whole Container, followed by reinstalling macOS and restoring from a backup. Unfortunately Apple's instructions to reinstall macOS are a bit vague here, but I suspect they really mean a clean install versus just reinstalling macOS over top of itself which won't fix any file system issues.
How to reinstall macOS - Apple Support
I've recently seen a post on here where the user mentioned the issue was caused by bad memory (determined by Apple during a repair of their Mac). A failing drive could also be the problem as well as mentioned by @Barney-15E.
Make sure to have a good backup before attempting anything.
You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.