In the MM Customizing (Transaction OMF2) you define that the valuation price is mandatory. You then create an order that is immediately relevant to materials planning or you release it so that purchase requisitions are generated immediately. If you then add a service operation with a service package to the order but without specifying the gross price, the system branches to the purchase requisition and requests you to enter the valuation price. After you enter the price and press ENTER, you branch into the order again. The price from the purchase requisition is displayed there in the operation header.
If you now want to save the order, the system again branches to the purchase requisition and you are again requested to enter the valuation price. Finally it is not possible to generate/update the purchase requisition.
Since the gross prices of the service lines create the totals of the operation price, the value entered in the purchase requisition is always overwritten with the initial price. This is not accepted in the required entry field check. The MM always transfers the price from the purchase requisition to the operation header. In theory, each requisition item can have several service lines attached. Thus there is no guarantee that the price of the requisition item is transferred to the correct service.
So define the gross price also as a required entry field in the service Customizing.