Your
Scene_Equip is fubared in short. Never ever edit or remove the default code. Create an empty section above main and you can override any code found above. This way the default code stays intact. Until that is, you're absolutely sure your code is finalized and ready to replace the original code. Back on task. It seems you removed the equip left and right window. The help window. But you still have code trying to use the right window's information elsewhere in the scene. This will generate errors as well. But in regards to the error you have. It is because of the following line in Scene_Equip:
@item_window1.index = @item_index
@item_index isn't defined within the code anywhere before that line so it is nil. You're are telling it that the index is nil. It's the reason that within Window_Selectable you get an error when comparing a undefined object against 0. Even if you change it back to @equip_index you will still have the errors I mentioned above. To move on with helping you I would need to know what you expect this Scene to do and look like.
Good luck with it Helomaa! :thumb: