When signing in to Skype for Business, we may encounter the error “Can't sign in to Skype for Business”.
Your account information for signing in may be correct and server services may be up. Skype for Business is known as a unified communications platform.
This issue occurs because the DNS records for Skype for Business are not added to your computer or cannot be found by the application, or because the Lync server that Skype for Business is trying to access to sign in is not responding. If you are experiencing this issue because the Lync server that Skype for Business is trying to access is actually not responding, the issue should resolve itself within a few hours and all you have to do is wait.
On the Skype for Business sign in page, follow the steps Gear – Personal
In the Advanced section, we need to go to the Manual Settings steps.
Enter sipdir.online.lync.com:443 as the Internal and External server and after confirming the process, you need to try the LOGIN process again.