Additional settings-related features and issues within Microsoft Teams Free
I had this issue, too.
The organization was not listed as a tenant in portal.azure.com. Thus, that was a dead end for me.
I contacted Microsoft Support and Erwyn helped with resolving the issue. He asked me to enable Teams Insider. I did so and then had to wait for and click on the upgrade and restart action in Microsoft Teams. Then I opened the Settings \ Accounts and Orgs screen and clicked on Decline on the unknown organization. This fixed the issue in the Desktop and Web client for Teams. I uninstalled and reinstalled Teams on my iPad and that resolved it there, too. Ideally, the Accept and Decline actions should appear on the personal profile, too.