iPhoto may not be able to burn a disc if the computer has more than one optical drive available.
When burning a disc from iPhoto, you are prompted to insert recordable media. Both optical drive trays may open. In this case, no matter which tray you insert the recordable media into, iPhoto does not find the inserted disc, but instead cancels the burning process.
To avoid this issue, insert your blank media into one of the drives before you select the option to burn a disc in iPhoto. Close any drive trays that are opened. If you are prompted by the Finder that you have inserted blank media, click Ignore in the dialog that appears. Then select the option to burn in iPhoto.