I have installed Lightroom v 5.6 (latest) on my desktop computer and on my Laptop. I have made a book. I get up one exclamation mark at the pictures and the text "this photo is offline or cannot be found. It will be omitted when you print your book» This I get on both the PC. Do not have the programs up and running at the same time. I'm not offline and I have brought in the directory where the images are located on, which is in Onedrive. Is it not possible to use OneDrive, so I can pick up my book both from my desktop and laptop computers? Can someone help me?
The "cannot be found" message means you have moved, renamed or deleted the photo (or the folder that contains it) outside of Lightroom. (Hint: don't do this any more)
In the Library module, the photo should have an exclamation point icon, as shown here in Figure 2. That same link has instructions on how to fix this problem, just click on the exclamation point icon and then click on Locate and find the photo.