The cause of the error is Price List not setup for the Inventory items entered in the SOP line window.
Resolution:
1. Go to the Item Price List Maintenance window (Cards-->Inventory-->Price List).
2. Enter Item number, Price Method, Default UofM, Default Price Level and enter the price details as well in the scrolling window and Save the information.
3. After completed the step 2, go to SOP entry widow delete the line item and re-enter the same items to check whether the error persist or not.
Hope this helps!!!