You’re likely trying to add a user to a template plan, but users cannot be added directly to template plans.
As explained in How to Create a Plan With the Soma Analytics Plan Builder and How to Create a Template Plan With the Soma Analytics Plan Builder, this restriction is a built-in safety measure to prevent unintended updates.
If users were assigned to a master template, any changes made to the template would automatically apply to all associated users, which could lead to unintended consequences.
To learn how to properly assign users, refer to: