It may be the mesh is not connected. You can confirm this by dropping a Modal analysis on the Model cell of Static Structural in Workbench then in Mechanical, drag the Fixed Support down and drop it on the Modal branch. In Analysis Settings for Modal, request 12 modes and Solve. If you find the first 6 modes are practically zero, you have an unconnected mesh. Set the display displacement scale factor to 0 and use the Total Deformation plot to see where the connection is broken. Or the animation might also show you where it is broken.
Try to use the Mesh Merge command to merge two coincident nodes so that a connection is established.