Looks like a non-manifold mesh - aka has holes. I had this too - and started over. But try these steps 1st.
In edit mode, select all, hit M, Merge Vertices by Distance to confirm. Good chance there are overlapping vertices or surfaces.
Or, try the 3d print add-on to repair the mesh.
Under clean up, you can try to repair the mesh. “Make Manifold”
Example (you can do this in Object mode too)
Fixed
