Creating restricted channels within a team for confidential discussions or projects
Hi @Jenny G
Based on my research, it looks like only Basic Plans are currently supported in Teams Private and Shared Channels, while Premium Planner capabilities will be added at a later date. If the plan you're trying to add is a Premium Plan and the destination is a Private Channel, this may explain why it does not appear when you attempt to add it to the channel.
You can find more information in MC1262590 - Planner tab support for Shared and Private Channels in Microsoft Teams
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Please feel free to correct me if I misunderstood your request. If you have any additional concerns, feel free to comment below. I would be more than happy to assist.