can't simply transfert videos to itunes

i have iPhone6 wired in, itunes on its lastest version (12.3.2.35) on windows7

my computer is a dell optiplex 755 core2duo with 2 GPUs (added one)
i can't just add videos to itunes, i used to get an error message that i don't remember, i clicked on "never show again" and that was the last time i see it !

now i just drag and drop and it simply nothing happens, i also try add to library also negative.
i tried so many free and paid converting softwares to make sure that is not a compatibility problem, still nothing happens

mp3 simply gets added by drag and drop works pretty fine

i unticked the checkbox in "general settings" that's called "manually manage music and vids" so i can just not have to keep old music on computer whenever i want to add a new one.

thanks for providing any kind of help