We have seen that error happen when there is a kit that is not fully committed. For example: the SO line has 100 qty of the kit but the first component only has 90 available, so 90 of the kit is committed. When you fulfill, the first component will have a qty of 90, because that's all that is available. The second component will have a qty of 100, because there is enough available. Changing the second component on the fulfillment to a Qty of 90 should clear the error.