After activated 4K Video Downloader, only 51 video can be downloaded in 1 playlist

As the topic, after I buy the key, and download the playlist, only 51 videos in the playlist can be downloaded in that playlist, but your products said this product can download unlimited video under 1 control from 1 playlist. But I don't think so, please reply me, and solve this problem. Thank You !

This is the playlist that I want to donwload : https://www.youtube.com/watch?v=vpbZW07z-Zk&list=RDvpbZW07z-Zk