Copy Planned Delivery Time to Activity Duration does not work

Hello Experts

I'm trying to copy planned delivery time of a material component into the activity duration and it doesn't work

Procedure:

• I have an external activity and the network is backward scheduling
• The activity has not yet been scheduled but the end dates for the Project & Network are in the far future
  • some times I give it a duration and the end & start dates are also far in the future
• I assign a material component to an external activity using CJ20N
• I set procurement type to "PR+Res. for WBS element" (or anything with a PR sometimes)
• I enter Quantity, Vendor, P.Org and the Align. finish date is already ticked (I made sure of that in configuration)
• Planned Delivery Time is inherited automatically from the material master to the material component (or I enter it manually sometimes)
• I go to component overview and choose "select all" (sometimes it's just 1 entry)
• I go to the menu bar -->edit-->component-->transfer delivery time -> duration
• I get the message "Maximum of 20(30) days copied as duration of activity 0030" / Message no. CN747
  • I know this message is because they are 20 working days out of a total of 30 days according to the factory calender
• I go to the activity (external tab). planned delivery time field is not populated
  • in case I had already put a value in it earlier, it does not change even though it is less than the planned delivery time for the component.
    
• I go to the activity (Dates tab). duration is still 0
  • in case I had already put a duration earlier, duration does not change at all even though it is less than the planned delivery time for the component.

the system responds as if I didn't do anything

can anyone help me please !!

Am I doing something wrong ?? can anyone tell me the procedure that works for him/her ??

Is there any configuration, business function activation or SAP Component installation to solve this?

Thanks for reading