So, I’m not totally clear on which version you are currently using, and which version you are trying to upgrade to. You say that you currently have 12.0.3, which sounds fine. Then which version do you want to upgrade to? 12.0.6 does not exist as mentioned earlier.
From the error message, it sounds like maybe something is getting lost in translation here, and really you have 11.0.6 and are trying to upgrade to 12.0.3.
In that case, check out this thread which may or may not help you: Cannot manually upgrade from 11.0.3