Each customer account can only have one active subscription covering a given delivery date. If the manager already has an active plan and tries to buy another one for themselves, the checkout stops with You are trying to create a subscription on a date you already have an active plan. When account members are enabled the message adds: If you are trying to create a subscription for another person try creating a membership. — meaning: add that person as a member on Dashboard > Members, then select the member before choosing the plan so the subscription is created on the member's account instead of the manager's.