Hi Tom,

re:
"P.S. What does "Out of attachment space" mean?"

File attachments on messages in the discussion forum is limited to 50MB per user, to avoid having the web server's storage getting completely filled.

If you go to Options > Attachments you can look for some older attachments that you could delete to free up some space or you could upload the file you want to share to a file sharing service like dropbox / onedrive / we transfer / etc... and share the link here instead of attaching the whole file here in the forum.

- Michael