If an upload stalls or fails:
Check your connection: uploads are large, and switching between Wi-Fi and cellular mid-upload can break them. Retry on stable Wi-Fi.
Make sure the file is an MP4 video: that's the format deliverable uploads expect.
Free up a little phone storage and retry: iOS can cancel uploads when storage is critically low.
Force-quit the app and try again: the submission isn't created until the upload completes, so retrying is always safe.
If it fails repeatedly, message support from Settings → Help & Support with the deal name and roughly how large the video file is.