I tested in my lab with a Gmail account (invited as guest to a tenant) and received the same error.
I suppose the cause of this issue is this add-on doesn't support guest accounts.
It is not possible to login to the tenant as a guest so it would default to the personal account which cannot access the application and return the error message.
According to this link, you may need a Microsoft work or school account.
If you have a Microsoft work or school account, install the Teams meeting add-on to your Google Workspace to be able to schedule and join Teams meetings directly from your Google calendar.
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