Not able to set approval date dynamically

Question

I am trying to create a workflow process in dynamics 365 where "approved on" field is automatically populated when the expense request is approved. I followed the following steps:

1. Selected my app from Solutions and switched to classic UI.
2. Created a new process (https://i.stack.imgur.com/9lcLZ.png)
3. Added condition where it checks if Expense request is approved (https://i.stack.imgur.com/xnPWa.png)
4. The following window pops up when I click on set properties (https://i.stack.imgur.com/eXUd3.png)
5. I need to select 'Approval date' field and use form assistant and set a dynamic value but not able to do so. Am I missing something?

Tried cross checking all the steps and clicking on fields