question

Hi @zoecooke-8362，

Does anyone else have the same issue in your organization?

To narrow down this issue, we firstly recommend you login in Teams web client to see if you can add comment normally. Also, you can try to see if you can add comment directly when access Planner directly(https://support.microsoft.com/en-us/office/sign-in-to-microsoft-planner-fe43c972-5a95-4071-86d4-423a64a3b21e?redirectSourcePath=%252fen-us%252farticle%252fsign-in-to-microsoft-planner-90f10431-2b40-45c5-a2f5-55bc23af6ad8&ui=en-US&rs=en-US&ad=US).

In my experience, you may not be able to comment on tasks in Planner if your organization is not using Exchange Online for your account. For more details, you can refer to:
https://docs.microsoft.com/en-us/office365/planner/planner-for-admins?redirectSourcePath=%252fen-us%252foffice%252f9652e4c7-48e3-4dad-9e71-0c783ec3d0f8#can-people-in-my-organization-use-planner-if-they-dont-have-an-exchange-online-mailbox

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