Hi @UtauyoNASE
As above said, you can log in Microsoft Teams Admin Center by this link: https://admin.teams.microsoft.com/teams/manage.
You can turn on “User can communicate with other Skype for Business and Teams users” to enable external access as the following image.
By default, external access is turned on in Teams, which means that your organization can communicate with all external domains. For more details, please refer to this link: https://learn.microsoft.com/en-us/microsoftteams/manage-external-access.
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