Additional features, settings, or issues not covered by specific Microsoft Teams categories
Hello MarcB21_21,
Do you have phone system license in your Office 365 tenant? If not, you won't have that button in Teams admin center.
From the official article, you need to connect a holiday to an auto attendant. And phone system in Office 365 provides auto attendant.
https://docs.microsoft.com/en-us/microsoftteams/what-are-phone-system-auto-attendants