First, you have to enable monetization on your YouTube channel to show that you are intrested in earning money from your videos.
Go to YouTube settings -> Go to account monetization -> Enable monetization -> Accept and agree to terms. Your account will be reviewed for monetization, you should get a confirmation email in less than 24 hours.
After monetization has been enabled, you can now link your channel to Adsense.
Click "associate an Adsense account" -> select Google account -> Simple description the content of your videos -> submit the Adsense application after filling all the required details. Your Adsense application will be reviewed and you will receive an email when your application has been approved.
Linking YouTube to AdSense allows sponsors to place ads on your videos. When people view these ads, you generate revenue. To link YouTube to AdSense you first need to monetize your account, which shows that you are interested in earning. Once you have logged in, click the monetization button in settings to enable it. Once enabled you need to access the AdSense option directly below it.
Find the ‘associate an AdSense account’ and click ‘next’. Select the google account that you want to link with AdSense. Then describe your content simply and fully to allow advertisers to understand the type of videos you are making. Click submit application and wait for approval.