Hello Willie.
If you receive this message, it means that you have reached the threshold set in Preferences> Undo History menu, and it’s notifying you that some entries will be deleted to free up space. Remember that a project file saves ALL tools and subtools loaded into it, and each subtool has its own undo history. This can add up quickly. If you don"t require all those extra tools loaded into the scene, consider only working on your active tool as a ZTL (tool) file. This format only saves the active tool.
Also remember that you must store the config (Preferences> Config> Store config) when making changes to the preferences in order for Zbrush to remember them at startup.
If you’re saying that this happens when it shouldn’t, for instance in a brand new session without many undos , then there may be an issue with your installation and you will need to contact support. If it’s only happening in that file, then the simplest explanation is that your file contains an undo history that exceeds the preference settings.