Microsoft Skype for Business on Mac displays a red error message that states:

We couldn't sign you in. Check your account info and try again. If your admin provided a user name, you might need to enter it in Advanced Options.

In addition to the error referenced above, you may experience the following symptoms:

• Your conversation history is not being saved in the "Conversation History" in your TechMail mailbox as usual.
• You cannot save Skype for Business conversations to an email message.
• You cannot send mail to a Skype for Business contact.
• You cannot search personal Outlook contacts from Skype for Business.
• When your account is viewed by an IT support professional using the Active Directory Users and Computers MMC snap-in, the E-Mail field may not be populated, and your account may not have an Exchange license.

You may be either not correctly connected to your TechMail account or may have been locked out of your TTU domain account.

You also may have incorrect credentials stored in a keychain in macOS or iCloud.

1) Make sure that Outlook is set up correctly and is connected to your TechMail account.

2) Remove saved credentials from the Passwords app and from Keychain Access that might be in use but are not current/correct. Outlook or Skype for Business will prompt you again for them, if needed.

Outcome

Skype for Business is successfully connected to your TechMail account again.