Try something like this:
Condition: motion active
Rule: motion active
Action for true: dim lamp to 80
Action for false: delayed dim lamp to 20, cancellable
More options: only when mode is Evening, Daytime
When you have both triggers and conditions, you have a triggered rule, which is only evaluated when the trigger fires.
You want a rule, which is evaluated whenever the state of the conditions change. That way, we catch the motion-inactive event, and use it to start a timer. The timer is cancelled if motion becomes active again.
When you have conditions, there must be a rule to define the relationship between those conditions. But when there is only one condition, Rule Machine automatically gives you the trivial rule of that condition.