Sales Order Error Item and UOM not on price list

1. Please check whether you have defined the Item on the Price List (which you are using in SO Header) or not. 
2. On the SAles order header first check the price list in use. 
   Then enter the Item and price in that particular pricelist. 
   Eg:You note Price list XYZ from Sales order header. 
   Go to define price list window, qury for XYZ. 
   Check if the Item you are using is available in the pricelist. 
   If not plese enter the Item and save 
3. Previously the Item it was defined on the Price List (which using in SO Header) 
4. Check the Start Date and End Date for that Item in the Price List.