Hi @Calven_Chow ,
I think the 'Assign to' filed is a person/group field which allows multiple selections, right?
Please try to remove the 'Apply to each 2' action and add the Select and 'Append to array variable' again.
The point is to input the expression manually instead of using the dynamic content.
In you case, try these expression:
Select From: items('Apply_to_each')?['Assign to']
Select Map: item()?['DisplayName']
Besides, I found that you hadn't click on the button of the select action, please click it first and then fill in the Map parameter.

Hope it makes sense.
Best Regards,
Wearsky