Thanks for reaching out. You should've received an email with the reason for disapproval and can also notice an option to appeal for our team to take a second look if you feel it isn't valid. Hope this gets resolved.
While we can't share specifics. You should have received an email and a notification with more info in Creator Studio. Take a look at those resources and note that you can always reapply for the YouTube Partner Program again in 30 days.