Disk Utility - Operation successful, but says warning: bst_node_limit_reached

There are two types of snapshots that may be stored on the startup volume. They are Time Machine and APFS snapshots. The former may appear as the result of having Time Machine enabled but not backup drive connected at the scheduled backup time. The latter are created by the APFS disk control software. They are used to facilitate certain disk operations such as file moves or disk restore. These are somewhat related to the assistance provided by journaling in HFS+. Both types of snapshots are supposed to be purgeable. That means when disk space is getting to the least space threshold, snapshots will be deleted or purged to make more room on the drive. See What Is Purgeable Space in macOS?

Errors reported about snapshots do not affect the ability of the OS to read, write or open, save files kept in the allocated user-space of the drive. So, these errors should be ignored when they are reported during a First Aid scan.

At present, you can only stop Time Machine snapshots. However, the easiest way to prevent them from occurring is to turn off Time Machine whenever you disconnect the computer from the Time Machine backup drive.

Time Machine Snapshots

Open the Terminal in the Utilities folder and enter or paste the appropriate command line (only the part in italics.) Press RETURN and enter your admin password when prompted. It will not be echoed. Press RETURN again.

     To turn them ON: sudo tmutil enable

     To turn them OFF: sudo tmutil disable

Note that turning them OFF will also delete all existing snapshots. For more about snapshots see: About Time Machine local snapshots.

APFS Snapshots

Unfortunately, macOS does not provide user control of APFS snapshots. However, they can be turned on or off using a third-party utility called Carbon Copy Cloner. CCC can be used to copy and sync files as well as make bootable clones. It is the "go to" utility for such tasks. CCC does provide switch control of APFS snapshots as well as snapshot management. It is not recommended for users without some extensive knowledge of APFS. Personally, I leave them alone.

APFS snapshots also occur when macOS performs an OS update (or when software is installed I think). macOS does this in case the install causes a problem so the user can revert back to the way the system was before the update/software install occurred. Some apps may also utilize APFS snapshots with the most likely apps being backup apps such as Carbon Copy Cloner.

Typically APFS snapshots are supposed to be automatically deleted after about a week. I don't know if this also goes for snapshots initiated by third party apps. I have seen a few posts on here which indicated that sometimes these APFS snapshots are not automatically removed in a timely manner.

Here is an article on how to thin or delete APFS snapshots:

https://derflounder.wordpress.com/2018/04/07/reclaiming-drive-space-by-thinning-apple-file-system-snapshot-backups

FYI, if you notice unfixed errors that are not part of a snapshot, then I recommend you clone the drive and erase it before restoring the clone. I've seen First Aid report everything is "Ok", but the "Details" showed unfixed APFS errors which did affect the performance of our Macs and even prevented one of them from booting to the internal drive and even caused a Kernel Panic on another system when trying to mount the system in Target Disk Mode.