>>Do you have an M365 plan which includes Teams? Is it a business tenancy or some sort of personal/family plan?
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>>If you have an M365 plan that includes Teams is the M365 user account you're signing in as provisioned with Teams? Bear in mind that for a long time MS was offering Teams for free i.e. if you had an Apps for Business plan (apps only, no online services) you could still use Teams free trial. However, that stopped around October last year so now you need a plan that explicitly includes Teams.
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>>I've never had to worry about installing the Teams add-in for Outlook. With my own business tenancy, and the client business tenancies I support we all have plans that include Teams and have the Teams desktop app installed. With that combination the "Microsoft Teams Meeting Add-in for Microsoft Office" just appears in Outlook, no separate installation required.
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>My plan is Exchange Online (Plan 1). So, this does not support it. Thanks for the useful information

Exchange Online (Plan 1) is $5.10 per month. Upgrading to Microsoft 365 Business Basic ($6.40 per month) adds Teams, SharePoint, OneDrive and other features.