This looks like split edges on the loop where the division in the shell appears. Select all in Edit Mode, M > Merge > By Distance. See if that works.
– Robin BettsOct 16 '23 at 16:16
@RobinBetts hi yes, this has solved the problem. thank you very much! is there any reason this happens? as the first time I did it, it worked alright, the second time had a split.
– Charles LohOct 16 '23 at 16:28
I can't tell, of course, but a very common cause is imagining that a right-click on an extrusion cancels the extrusion. It doesn't. It cancels the move, leaving a 0-length extrusion (invisibly) in place. Ctrl-Z is needed to get rid of the recently-created faces.
– Robin BettsOct 16 '23 at 17:15