If the download feature in Rufus 3.5+ doesn't work, download the Ubuntu .iso file manually from the official ubuntu.com website, and then browse in Rufus to the Ubuntu .iso file that you downloaded instead of downloading it from Rufus.
If the download feature in Rufus 3.5+ doesn't work, download the Ubuntu .iso file manually from the official ubuntu.com website, and then browse in Rufus to the Ubuntu .iso file that you downloaded instead of downloading it from Rufus.