The probable reason is that the file is located in a folder that is being synchronised to cloud storage. Examples of this would be syncing to Google Drive, Dropbox or OneDrive.
In such situations, the file can become locked while it is being synced and this makes it appear as though it is already open when you try to open it in FlashBack. If you move the file to a folder that is not synchronised (i.e. move it out of Dropbox, Drive, etc), you should then be able to open it without any problems.