Custom action context for order
So the context is mandatory and it is actually important because it sets the data you want to fetch when the rule is triggered.

I just needed to set the context to the good entity as follow.

"entity" = @ContextDefinition("entity:uc_order",


The working annotation is the following.

/**
 * Definition of the action performed.
 *
 * @RulesAction(
 *   id = "my_action_id",
 *   label = @Translation("My action label"),
 *   category = @Translation("My module"),
 *   context = {
 *     "entity" = @ContextDefinition("entity:uc_order",
 *       label = @Translation("Entity"),
 *       description = @Translation("Specifies the entity, which should be saved permanently.")
 *     )
 *   }
 * )
 */


Then you are able to fetch the current uc_order in the administrative page.