409
fees.version_not_activatablefeesRule R11This version cannot be activated
Activation takes a draft version to active. Activating an already-active version is a no-op that returns the plan and emits nothing; activating a retired one is refused.
Why the API refuses
WHEN ActivatePlanVersion is executed on a draft version, THE SYSTEM SHALL set its status to active and emit FeePlanActivated; WHEN it is executed on a version that is already active, THE SYSTEM SHALL return the plan unchanged and emit nothing; IF the version is retired THEN THE SYSTEM SHALL reject it.
What to do
Create a new draft from the retired version rather than reviving it.