Making one wall invisible?

there are different ways of achieving this. It all depends on what is the purpose of it: You can "hide" a wall or any other element by view. Just select it, right-click, choose "hide in view", then element.
You can also demolish the element, or move it to a Phase, or even make it part of a design option.
Again, it all depends on what's the purpose of hiding it.
Hope this helps.

You can also right click on the wall and then select "Override Graphics in View... by Element".

Do you mean invisible or transparent? For transparency: Right click > "Override" > "Transparent".

Visibility Graphics allows you to make all walls invisible.