Why can't I download YouTube videos?

When ever I download any video from you-tube from IDM this (shown in picture) appears,
I don't wna change the down-loader as I m an IDM lover but I have tried on different browsers ,and my down-loader is also up to date ...
please help
THANKX !

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Although it is illegal to download some videos from youtube, what browser are you using & what video download addon are you using & a link to the video would help.
 
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