Accessing Microsoft Teams Free using personal or organizational credentials
What exactly is the iOS version of your iPad?
Be aware that the New Teams application is officially supported on the 2 last major iPad iOS versions.
https://learn.microsoft.com/en-us/microsoftteams/teams-client-system-requirements
To be sure that you are not on an outdated Teams version which only supports organizational accounts, uninstall any currently installed Teams app and install the latest New Teams application from Apple Store.
https://www.microsoft.com/en-us/microsoft-teams/download-app