Just a guess, but if you are trying to upload it to the site directly, it will be too large a file for that. Have you tried uploading it to YouTube, then linking to that by using the "Insert" feature here?
So you are trying to upload it directly to the forum here? I don't know of any forums that will allow this, as they have to pay for server usage & if everyone was posting large files, it would soon cost them a fortune.
If you haven't already, upload it to YouTube & link to it from there, using the "Insert" feature in the section where you post a message. It's the bit with the 3 dots & the arrow pointing down, in between the smiley face & the camera.