An action is the part of a CAPA that changes something. Everything before it is analysis and everything after it is verification - this is where the process, the document or the equipment is actually altered.
Adding and completing actions needs manage_capa_actions.
Planning them
Each action carries an owner and a due date, because an action with neither is a wish. Corrective actions fix this cause; preventive ones stop it appearing somewhere it has not yet.

Add one action per thing that will change - not one action called "fix the process".
Give each an owner who can actually do it, and a date they agreed to.
Start the CAPA when the plan is complete: actions cannot be completed while it is still being planned.
Let owners complete their own actions, with evidence.
Completing one
Completing asks what was done, not just that it is done. The evidence note is what makes the action checkable by someone who was not involved.

When the last open action is completed the CAPA moves itself to the effectiveness stage. That is deliberate - work finishing is what makes verification due - but it means you cannot leave a CAPA parked in progress once the plan is done.