This error message actually is a sign of the short of the system resources. In other words, most likely, the computer is out of memory when you performed one action.

This is usually because you do not have enough memory on your computer or you have processed too many high resolution images in this book. For instance, for the 12" x 9" size page, the biggest file size can be 3600 pixel x 2700 pixel, any images greater than this size can be a problem.

To fix this error, sometimes you just need to close the program first, and then reopen it. Most of the times, this process will free up most of the computer resources, and you should be able to continue editing the book. Or you can even try rebooting the computer.

If this message keeps popping up, you will have to check all the big images/photos added into the book, re-edit the bigger ones to decrease the file size.