Finalizing a movie is not a step that you "must do". It is a convenience that renders the movie in all available sizes all at once, so when you go to save it to YouTube or iTunes later, it happens much quicker. If you don't need all sizes, you can just go to the Share menu and Share in the size you need.
When you Share to Youtube, you can get errors in two ways.
1) There was a problem with rendering. This should not happen if you finalized and waited until it finished (could be an hour or more)
2) There is a problem connecting to YouTube. This is usually caused by using the wrong User ID and Password for YouTube. A couple of years ago, YouTube changed the way you log in. You now usually have to use your GMail email address and password to log in to YouTube.
To see if your movie finalized correctly, try SHARE/TO iTUNES in a size you like. Then see if you see your movie in iTunes and can play it. If so, you know it rendered (finalized) OK. And it is more likely that #2 above is your issue. However, if you get an error message when sharing to iTunes, it may indicate a different problem.
There is a more fiddly way to determine if you previous "Finalize" completed correctly. Do this only if you are comfortable navigating through a Mac's file system, because you run a risk of harming your Project if you don't do it right...
You can go to your Project in the Finder. Go to Movies/iMovie Projects/<your project name>. Right-click on the Project Name. Select Show Package Contents. Go to the Movies folder within the package. There you should see your finalized movies. Pick one and tap the space bar. It should start playing. That tells you that the Finalization worked correctly.
If you tried the Share to iTunes suggesstion above, it would have found this copy in the Project Package and copied it to iTunes, so it should have been very quick to get to iTunes.
You could copy one of these movies in the size you want and paste it on your desktop. Then try to upload through the YouTube website.