Hi @Caspar - ABO
According to our official documentation, users with on-premises hosted mailboxes must be synchronized with Microsoft Entra ID. In addition, they could manage meetings if the requirements listed on Requirements for mailboxes hosted on-premises section are met.
After meeting the above requirements, make sure the integration between Teams and Exchange is configured correctly, even in an on-premises environment. If you still can't schedule meetings, please log in to the Teams admin center as an administrator, where you can manage and assign user meeting policies to allow users to create meetings.
Alternatively, you can schedule meetings through Outlook.
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