This is posted in the wrong forum, I have moved it.
Go to:
Admin panel -> Settings -> Attachments
1. ensure that "Allow Attachments:" is checked.
2. under "Online Attachments:", ensure that "Allow online attachments upload" is checked.
3. make sure that the file you are uploading is not greater than "Maximum File Size:" (although it should give you an error if it is)
4. Ensure that "Attachment Folder:" is the full system path, NOT a URL.
5. make sure that the file you are uploading is one of the types listed in "Accepted File Types:"