Skype for Business, Login Troubleshooting Steps
Why can't I log into my Skype for Business client?
If you aren't sure if you have an active Skype for Business account, you can check that here: https://uc.illinois.edu/
If your Skype for Business account is active, please use the "Skype for Business Sign-In Address" listed on the webpage to sign in to Skype for Business.
Make sure that you have at least the minimum version of the Skype for Business client software installed: Skype for Business, Minimum Client Versions Required
If this is a Windows 7 or later machine you should also make sure you try clearing your Windows Credential manager of any stored Skype for Business credentials. This can be done by going Start->Control Panel and searching for Credential Manager. Once you are in Credential Manager clear out all the stored Credentials that are saved in there by selecting them and choosing remove from vault. Skype for Business credentials should have OC1 in the title but clearing all credentials saved in it, just to be sure, shouldn't cause any issues.
If this is a Mac, please check Keychain Access for any stored passwords for Skype for Business and delete any you find.
Please test sign-in again after clearing the credential manager.
If this still does not work you should try to sign in on a different machine. If you are able to do that then there is a problem with the local client on your computer and should try uninstalling and reinstalling the Skype for Business client, or contact your local IT support if this is for a University machine.