Issue
Skype for Business fails to sign in for Office / Microsoft 365 customer.
Can't sign in to Skype for Business
Skype for Business couldn't find a Skype for Business Server for <domain.com>. There might be an issue with the Domain Name System (DNS) configuration for your domain. See KB2566790 for details and contact your system admin.
Resolution
The issue was caused by missing Skype / Lync related DNS records for the domain in question. After adding DNS entries as displayed below users could log in to Skype for Business with no issues.
August 2020
Microsoft Office 365
Skype for Business
How to add in DNS records?