Gmail Can't Send Attachments: How to Fix It

Try these in order

1. Check the file size. Personal Gmail accounts have a 25 MB attachment limit. If your files add up to more than that, Gmail automatically removes the attachment and inserts a Google Drive link instead (Google Help).
2. Make sure it isn't a blocked file type. Gmail won't let you attach executable files such as anything ending in .exe, because they're a virus risk — zip it or share it via Drive instead (Google Help).
3. Try a different browser and turn off any proxy. If attachments won't upload on desktop, confirm you're on a supported browser, test the same attachment in another browser, and disable any web browser proxy you've configured (Google Help).
4. Wait for uploads on your other devices to finish. The "Gmail's adding files" message means files are still uploading from another device — either wait for it to finish, or remove those attachments so the message can go now (Google Help).
5. Free up Google storage. Attachments count against the storage shared by Drive, Photos and Gmail, and once you hit the limit you can't send or receive attachments at all. Delete big old attachments and empty Trash and Spam (Google Help).
6. Remove and re-add the attachment. A "Couldn't send message" error usually means a file failed to upload to Drive because of low space or a slow connection — removing and re-attaching the file often gets it through (Google Help).
7. On mobile, check that Gmail can sync at all. If mail is stuck in send generally, confirm you're online and that Sync Gmail is ticked under Menu > Settings > [your account] (Google Help).