I think you're missing another possibility (and some other posters are too): maybe it is the case that a simulation can produce consciousness, by simulating a brain for example, but that it takes a sufficiently powerful model of computation to do so.

So, consciousness being computable may not necessarily imply that any given computer could do so. One potential way for this to be the case is if consciousness is tied to some form of hypercomputation (https://en.wikipedia.org/wiki/Hypercomputation), which can provide outputs that are not turing-computable. This would mean that a traditional computer couldn't simulate consciousness, but that another hypothetical machine might be able to.