What this is telling you is the ppi value is missing from the metadata. Because it is in the metadata there is nothing to worry about. It can be added at any time with photoshop.
1)Open the image in Photoshop
3)Type in the correct ppi
5)Save file to disk File>Save or if you prefer to use another name to protect the original use File>Save As then type in a name and click OK.
The main information to protect is the pixel dimensions that pertains to the data it self. If these values change so does the data. To protect the pixel dimensions uncheck resample image if it is checked in the Image Size dialog box.