First, I think you mean PPI not DPI. Pixels per inch.
If you don't have enough pixels in your photo, then Blurb will give you that message. This happens if you try to enlarge a photo too much. So, if the area of the photo when printed is going to be, for example, 6x4, and your image is 600x400 pixels, that's 100 pixels per inch (you do the math). Any image with under 200 ppi will be flagged by Blurb because it will print poorly, and so Blurb doesn't let you print it. The fact that the photo itself may have a 500 ppi number in the file itself is IRRELEVANT and that number means nothing and is not used. The calculated ppi (in my example, 100 ppi) is used.
You need to find a bigger (more pixels) image to use in that cell in the book layout, or make the cell smaller.
Thank you for the explanation.
I will try to find photos fitting the celles.
Blurb request that images be 300 DPI at the actual size in the page layout. If you resize the images using the Export module it must be sized the same or larger than the actual size when placed on the page in the Book module. The actual placed size will vary dependent on the orientation, Zoom setting, or if 'Fit to Cell' is selected. Because of this it is very difficult to determine the "Resize to Fit" settings to use in the LR Export module.
Use your full-size LR edited camera images without any resizing. The Book module will resize them and then apply proper output sharpening to the page layout images in the PDF sent to Blurb.