我想试一下surf154单元的导荷载,建立了四个节点,四个节点用四个梁单元连接,然后选择这四个节点,生成surf154单元,却提示no surface elements were generated because no exterior face of the selected element set having all of its nodes selected is the correct dimensionality for the specified surfface type.求解这是什么意思?