Merge后，实体变为橙色，无法mesh

Merge后，实体变为橙色，无法mesh。提示如下“The selected regions cannot be meshed automatically using the assigned element shapes. They must either be partitioned, assigned a tet shape, or manually meshed using the Creat Bottom-Up mesh tool.”请问是什么原因导致的呢，改如何解决。
谢谢！