Instead of waiting for converting and uploading the youtube video by lingq, download youtube video as mp3 file.
And you can add mp3 in “edit page” of your lesson
If you have already had the video, you can convert here Convert audio and video to MP3
I am a little confused with your problem, please let me know if i got it wrong.
PS: when i tried to upload a video file mistakenly instead of mp3, i got the same warning message as u got.