YouTube has begun sending emails out to content producers on the changes that are coming next month to how they can monetize their channels. The email lays out the new requirements in order to have access to the ad monetizing tools and that access will end starting February 28, 2018 if you do not meet the requirements.
Those new YouTube requirements are pretty steep. You have to have 1000 subscribers but the higher hurdle is the requirement for 4000 watch time hours over the previous 12 months. This last requirement will eliminate many channels from being able to monetize.