PROBLEM
When you try to sign in to Skype for Business Online (formerly Lync Online) by using Lync 2010, you receive the following message:
When you click Download and install now?, you're prompted to download and install the Microsoft Online Services Sign-In Assistant.
To sign in, additional software is required.
SOLUTION
To work around this issue, install the Microsoft Online Services Sign-In Assistant, restart your computer, and then try to sign in to Skype for Business Online by using Lync 2010. To install the Microsoft Online Services Sign-In Assistant, go to the following Microsoft website:
MORE INFORMATION
This issue occurs if you try to sign in to Skype for Business Online by using Lync 2010 on an operating system that doesn't have the Microsoft Online Services Sign-In Assistant.
Still need help? Go to Microsoft Community.