Iāve run into that a few times, and usually the issue isnāt the MP4 itself but the codec inside it. A lot of sites only accept H.264 video with AAC audio, so if your file was exported with something else (HEVC/H.265 is a common one), it throws that ānot a videoā error. Re-exporting with more standard settings fixed it for me almost every time.
Another thing to check is file size and length limits. Some forums are picky and wonāt clearly say when a video is too big, they just fail the upload. Iāve also had uploads work after clearing the browser cache or trying a different browser, especially when the site uploader is buggy.
If you need to re-encode quickly, free video editing software can help you convert the clip to a more compatible format. Iāve used it mainly just to change export settings, not for actual editing, and it solved the issue fast.