There are several reasons why monetization might be disabled for your YouTube channel.
Account StatusIf you receive multiple copyright strikes and/or worldwide Content ID blocks, you may lose the ability to monetize your videos. Your account must be in good standing to regain monetization after six months. Please note that deleting videos with worldwide blocks may still result in disabling account monetization.
You may check your account settings for specific information regarding copyright strikes and worldwide Content ID blocks. You may be able to resolve these issues with counter-notification, retraction or dispute.
Commercial use rightsYouTube may also disable monetization if you do not provide adequate documentation proving that you own all the necessary commercial use rights to all visual and audio elements in your video(s).
Moreover, you may not be able to use your AdSense account with any other YouTube accounts.
If your AdSense account has been disabled for invalid click activity or other reasons, you cannot participate in account monetization. If your account is reinstated by AdSense, monetization will be restored within a few days.