In case you get an error saying for example
This video may be inappropriate for some users this means it is restricted from unauthorized access. The same would be if you open it in browser's private mode.
To get access to it in the browser you normally must authenticate yourself (i.e. log in to Youtube) so Youtube can check your identity and authorize you (i.e. give access) for viewing this video if you, say, 18 or older.
Youtube provides authentication possibility for third-party software, however program does not use this way since it anyway would not be allowed by Youtube. An other way is used instead: authentication can be "simulated" by borrowing browser cookies from Youtube.
Once again in other words. Doing this you give the program ability to pretend it is you so Youtube can give an access to the videos that you are able to access in the browser. Which means first: it won't give possibility to access anything that you do not have access to in the browser. Second: it (probably) gives the program possibility to act on behalf of given Youtube account. Although program won't do anything bad, you are informed now and are free to choose to ignore such trick if you are not comfortable with this knowledge.
Now to the point. The following instructions are designed for Firefox and Chrome but in other browsers it should be possible similar way.
Log in to Youtube if you have not done this yet. Then open this URL: https://www.youtube.com/favicon.ico. Actually you can use any Youtube page but with single image it will be easier to follow next steps.
Open Web Developer Tools (this can be done by pressing
Ctrl+Shift+I) and select
Network tab there.
F5 to reload image. This will cause its URL to appear in
Network tab. Right click on it and select
Copy Copy as cURL. There are two such commands in each browser (see screenshots) but in this case it does not really matter which command to use.
Now open program, go to menu
Tools Options Advanced, right click on
net.set_youtube_cookies and select
Paste from cURL. After applying these settings program should be able to perform authenticated download.
It is recommended to remove these cookies from the program once you have done downloading everything you needed.
Help Troubleshooting Authenticated download