So monetization section is gone from your video settings.
There are 3 main reasons for this.
First, check out your account standing in the channel settings. None of the videos should have copyright strikes. Make sure you really have no strikes by following this link. If you have strikes, you can try restoring sanity by opening a dispute. Check this article for details. If you have no strikes, let’s try monetizing the video right in the video manager. It gives out undefined error. So lets report this error by clicking Send feedback button. Don’t forget to highlight a comprehensive screenshot and attach a brief description of you actions and expected results. Wait for a reply. But let’s try to monetize this particular video by checking out Advanced actions and enabling monetization from there.
This time it says “Sorry monetization is not allowed for this video”, which is far more comprehensive statement than that undefined error, isn’t it? I used a rude word in the video title, even though initially the video was monetized, after a couple of days it was flagged down by Youtube automatic profanity filters. Since you can’t really show many family-friendly ads next to the “ass” word I had in the description, Youtube decided to disable monetization for this video altogether. I don’t know of a way, especially a quick way to restore monetization even after you clear out explicit words, so I opted to just reupload the video without explicit description this time and deleting the original one, loosing all views and likes.