Work through these checks in order. Most problems are solved by step 2.
Open the video in YouTube first. Private, deleted, age-restricted, or region-blocked videos cannot convert, and no converter can change that. If it plays fine in YouTube, move on.
Make sure you pasted the complete URL: youtube.com/watch, youtu.be, or a Shorts link. Links copied from embeds or with extra tracking parameters usually still work, but a truncated link will not.
Open an incognito or private window and try again. This rules out browser extensions, especially ad blockers, interfering with the conversion or the download button.
Very long videos take longer to process. See converting long videos for what to expect.
Try a different browser, or try one of our sister-site alternatives. If the same video fails everywhere, the video itself is the likely cause.
Most often the video itself. Open it in YouTube first to confirm it plays.
Check the download bar and Downloads folder. Browsers sometimes block unfamiliar downloads; choose Keep on the warning.