Please check geometry in SpaceClaim.
Select geometry in structure tree --> Use right click --> Select Check Geometry. (Or Tools - Fault Detection in DM)
The geometry should be error free to proceed.
If geometry has any errors, please modify/recreate geometry at those places.
Please check sizing you have used in Fluent Meshing.
Please check following videos
Fluent Meshing Watertight Workflow:
Please go through help manual for more details
How to access Ansys Online Help Document
Guidelines on the Student Community
Fluids Engineering Courses | Ansys Innovation Courses