First of all, make sure your youtube channel have at least 4,000 hours of watch time in the last year and at least 1,000 subscribers to qualify for monetization. This rule came into effect in early 2018 and is another way for YouTube to prioritize watch time (as opposed to view count, which was the previous emphasis of the platform).
“Watch time” is the average number of hours spent by users watching videos from your website. Because YouTube mostly wants to keep viewers as long as possible on the site, viewing time has become increasingly valuable to creators.
You can work on some things to ensure that the channel has the best chance
of monetization before you hit the “Enable” button to monetize your channel.
It can also take up to 30 days or more for YouTube to review your channel,
and if your application is denied, you have to wait another 30 days to apply,
so do not take any chances — you want revenue to start coming in as soon as possible.