Hi @Leen_Mohamed, thanks for posting on the Forum and welcome to the SimScale Community 
In addition to @nelta’s comments, there is an import option for STL files called ‘Optimize for LBM / PWC.’ When selected, the CAD model is imported as a faceted geometry, which is specifically tailored for use in PWC/LBM simulations - make sure it’s toggled off.
If you’d like us to take a closer look at your project, please post a project link (here’s a guide explaining how).
Cheers
Igor