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If your Outlook Teams Calendar plugin disappears, try to follow the instructions at:
https://support.microsoft.com/en-us/office/understanding-the-microsoft-teams-meeting-add-in-for-outlook-eligibility-issues-and-troubleshooting-bf7a1194-1ca1-4a17-a6a9-8498f899ad91
Run this a couple of times, choose the "Fix Teams Plugin" option when prompted, following the instructions to open Teams first, then Outlook.
If this doesn't work, the only known workaround is to use the web-based calendar at outlook.office.com and create your calendar item there.