There seems to be 2 different issues you are facing. You seem to have a bad installation of RealPlayer and a Downloader issue.
At what point in time do you get your "Loadlibrary failed with Error 87: The parameter is incorrect."
Please follow the instructions from the below link to resolve your issue (in case you have a bad installation).
Please note: If you have any playlists created, you will have to recreate them.
To resolve the downloader issue, please restart your computer and try again. If it still fails, can you please follow these steps and try again?
1. Open Internet Options from the search window.
2. Under General Tab, click Delete in Browsing History and follow prompts.
3. Close Internet Options.
Once you are done resetting your Internet Options, please follow these steps:
1. Using Chrome, Firefox or Edge, go to YouTube.com and play any video and click the Downloader button. (This action will still show "Unable to download" message)
2. Close all Windows
3. Restart your PC
Now try your video. If the issue persists, please let us know.