A transaction is out of balance
Before you can record an stock transfer, the total value of the components and the total value of the finished items (displayed in the Amount column) must be equal. If the average cost of any of the items has changed recently, the transaction probably will be out of balance, however.
Generally if this occurs you should adjust the amount that appears for the finished item, rather than the amounts that appear for the component items, since those amounts reflect previous purchase costs. When you change the amount in the Amounts column, the unit cost for the item will be updated automatically.