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 safeguard to prevent unintended updates.
If users were linked to a master template, any changes to the template would automatically update all associated users, which could cause unintended consequences.
To correctly assign users, refer to: