Hmm, this typically means your content/thumbnails are not original/auto-generated. You can make changes and always reapply after 30 days. Here's more about reused content & how you can avoid them in future videos: https://yt.be/help/AsbB . Hope this helps.
We'd like to take a look into it. Could you send a screenshot of the "Monetization" page from here: https://yt.be/help/aVDx ? Also, share the channel URL with us. Keep us posted.