That's an indication that there exists a value somewhere in the Lua "universe" that it does not know how to save in a restoreable format. It knows how to do this for every standard Lua object, but not non-standard custom objects (except ones that have had special handling specifically written for them in the plugin).
One example is an open file-handle object. If you do this in a Lua script and run it from game_start():
[code]f = io.open('test.dat', 'wb')[/code]
...this opens a file for writing, and saves the open file-handle object in global variable 'f'. When the time comes for the game to be saved (which is immediately - AGS needs to create a restart point as soon as the game begins, which works by creating a save-game) there is no meaningful way for the variable 'f' to be saved, so you will get the "not literally persistable" error. (So make sure, if you do want to read/write files from Lua, that you do not keep a reference to the file handle around!)
Another example would be objects created by Lua C libraries that come as a DLL (for example, LuaSocket sockets or LPEG patterns). Are you doing anything like that? If not, maybe there is a bug in the plugin and one of these unsaveable values is being created accidentally by the plugin itself. It's not a bug I remember ever encountering myself, though.