I have successfully downloaded photos into Lightroom 6 and have been able to develop them and even export them but all of a sudden in the process of developing or exporting, Lightroom gives the message, "File can not be found", or" Folder can not be found ". Since these photos are already in Lightroom, how can it not know where they are? And is this a problem with Lightroom, my computer, or what I've done? And what should I do about this to correct it?
The photos are never stored in Lightroom. They are stored on your hard disk somewhere, in the place you put them, or in the place where you instructed Lightroom to put them.
Lightroom is a database. When you import pohtos, a record is created in the database for each photo, containing the file name and location of each photo. When the photo is no longer in the location Lightroom is looking (because somehow the file or its folder was moved, renamed or deleted outside of Lightroom), Lightroom tells you that it can't find the photo (or the photo is missing).
So you have to find the photo, using your operating system. Once you have found the photo, you have to tell Lightroom where the photo is, and what its name is. The exact tools used in this step are described indetail here: Adobe Lightroom - Find moved or missing files and folders