hi abhijit,
well since you have entered 1 line item in the PR Item list under which you have 2 services.
The system by default would take the 1st and since your PO is already been created with the 1st so the system is not allowing you to take the 2nd. Thus I suggest you is add another line item into the PR
and maintain the 1st service under 1st PR line item and the 2nd service under the 2nd PR line item as shown below :
Regards,
Sagnik