sync issues with Code two

Are you using a 3rd party mail application?

Apple now requires you to use two-factor authentication (two-step verification where two-factor authentication is not available) and create an app specific password for 3rd party applications that access your iCloud account with the use of a password, you can initiate the two-factor authentication (two-step verification) set up and create app specific passwords at My Apple ID

Two-factor authentication is currently available to iCloud users (in most locations) with at least one device that's using iOS 9 or OS X El Capitan or later. If you can’t meet the criteria for two-factor authentication you can set up two-step verification which only requires a device registered with ‘Find my phone' or a phone number able to receive SMS messages.

Hi, this article may be helpful: https://www.codetwo.com/kb/icloud-outlook-synchronization-diagnostics/.