When you get a red X on a file in your Dropbox folder, this means the the file did not upload correctly to the Dropbox cloud, even though you might see an entry for it. There could be many reasons for this, but the most likely one is that you ran out of space in your folder in Dropbox. You will need to go into Dropbox and delete some or all of the files in your folder and then upload the file again.  You should see a green check when the file uploads completely with no errors.