This is a known issue impacting some Mac users since February 3, 2021 according to a Microsoft note.
You can check your Microsoft 365 Service Health page and you will see the Advisory TM241048 "Users may be unable to use the '@' function to add attendees when scheduling a meeting on Microsoft Teams".
User Impact: Users may be unable to use the '@' function to add attendees when scheduling a meeting on Microsoft Teams.
More info: As an alternative method, users are able to utilize this functionality using the Microsoft Teams web app.
Current status: The deployment of the fix to the remaining infrastructure is taking longer than expected. We're continuing to prepare for the fix to resume deployment which we expect may occur by our next communications update.
Scope of impact: Any user attempting to perform this action on a Mac OS device may be impacted.
Microsoft is already working on a fix, but unfortunately it's taking longer than expected.