A failed upload shows a red block in the Media Library headed An upload failed, with a Retry button. Retry first. Most failures are a dropped connection and succeed second time.
If it keeps failing, the message under the file name is the server's own reason. Here is what each one means.
File too large for your plan (max N MB)
Per-file caps are 200 MB on Hobby, 2 GB on Creator and 8 GB on Studio. Shorten or re-encode the clip, upgrade, or use the desktop app, where local files stream from disk and this cap does not apply.
Cloud storage is full
Your whole library is over the plan cap. The Media page in the dashboard shows what is using it and lets you delete safely. This has its own article.
The file cannot be rendered and converting it failed
We try to convert unusual formats automatically. When that fails you will be told the format and asked to re-export as H.264. That is the fix: re-export the file as H.264 with AAC audio and upload again.
Unknown or expired upload session, restart the upload
Large files upload in pieces, and that session lasts 6 hours and does not survive a server restart. Just start the upload again from the beginning.
Out-of-order chunk, or Upload incomplete
The pieces of a chunked upload arrived wrong. Retry the whole file. A flaky or very restrictive network is the usual cause, and a different network usually fixes it.
Too many uploads, slow down
A rate limit, not a quota. Wait a few seconds.
The one that matters most
An upload that failed leaves the clip only in that browser tab. Close the tab without retrying and the clip is blank when you reopen the project, showing ⚠ Media missing. The editor warns you before you leave and the panel says it in as many words: these files are only in this browser tab. Retry before you go.
If you lost one that way, the timing, effects and keyframes on the clip are all still there. Re-import the file and it drops straight back in.
What formats work
Video, images and audio broadly work: mp4, webm, mov, mkv, avi, m4v, mp3, wav, ogg, m4a, aac, flac, png, jpg, webp, gif, svg and avif. 3D models in .glb and .fbx upload too, but do not go on the timeline directly.
Duplicates
Uploading the exact same file twice does not use twice the storage. Identical files are stored once.