When someone reports that a share link does not work, it is usually one of a few things. Check these in order.
The link was revoked
A revoked link stops working for everyone. Open the video's share settings and confirm the link is still active. If it was revoked, create a new link and send that instead.
The link was copied incompletely
A link that is cut short does not open. Copy the link again from the share settings, paste it somewhere plain to confirm it is whole, then send it.
The video is still processing
A video that has just finished uploading may still be processing. Wait until it shows as ready in your library, then share it.
The viewer is on an old link
If you created a new link, the old one may still be floating around. Make sure the person is using the most recent link you sent.