This is a poorly worded effect, but according to this tweet from Jeremy Crawford,
Eldritch Cannon does let you spend a spell slot to create the cannon. That slot must be spent specifically on the cannon, not on something else.
So, according to this, the intended use of this ability is that when you make the second cannon, it counts as building a new one, so as described in their eldritch cannon ability,
Once you create a cannon, you can’t do so again until you finish a long rest or until you expend a spell slot of 1st level or higher
So, with this and the tweet at our usage, it would seem that the ability is intended to instead say that when you create a second cannon you must expend a spell slot to do so, or take a long rest.