I want to sum the values filled in the "original estimate" field of each subtask created from a Story, and see the sum in the "original estimate" field of this Story (main/parent issue).
I'm trying to configure a rule for that, but in the first step of the rule when you select the "Field value changed", I can't select the "Original Estimate" field because it isn't appear on the dropdown options.

I have seen other topics about this problem but I tried unsuccessful.
Can you help me please? It's easy to configure rules for another fields but it's going very frustrating with this one.