To use Decisions for agenda creation and meeting management inside Microsoft Teams private channels, an additional Microsoft Graph permission must be granted to the Decisions app.
Read and write items in all site collections (Sites.ReadWriteAll)
This permission allows the app to edit or delete documents and list items in all site collections on behalf of the signed-in user (documents/lists the user has access to).
Make sure to check the “Consent on behalf of your organization” checkbox when approving permissions:
Approving permissions on behalf of your organization can be done by a Global Administrator, Application Administrator or Cloud Application Administrator in your Azure AD.
A Decisions administrator can enable private channels and approve permissions here: