As of July 31, 2021 Microsoft is retiring Skype for Business.
Instant messaging service through Skype for Business will no longer be accessible.
Use Microsoft Teams chat for instant messaging. You can Get Started with Teams using the related article. IT Staff also offer Office Hours to assist with the transition to Teams; register using Actions.
The instant messaging replacement is Microsoft Teams chat. Teams extends the capabilities of Skype for Business, bringing together chat, meetings, calling, collaboration, app integration, and file storage into a single interface.
Teams, included in Office 365, can be used via the desktop app, web app with any web browser, or mobile app.