I get crazy with this wall, help is needed

Hello my Blender friends,

I recently started using Blender to design a few models for my hobby game.
It's supposed to be a warship shooter, and what goes with that?
Exactly one ship that you can control.

I already have model V1.0 ready, but it's kept very simple.
On a YT video, I saw how someone designed a battleship in 10 minutes and I was very impressed by it.

I also experimented a lot with extrude and insert but I keep getting the same problem that the center of the ship creates this line (or wall) and I just can't get rid of it. I watched the video 10 times, got Chatgtp to help me, but it didn't work.

I hope someone here can help me, otherwise I really don't know what to do.