The "similar to" part of that error message is likely because of your current username. In that situation, I would recommend thinking of a username that is different enough to not have that error displayed.
Does the capitalization really matter though. Renaming an account involves a lot of things on the backend. So I doubt staff would want to grant a name change just to correct the capitalization of a single character. Also keep in mind that it would count as your 1 rename. By just letting the capitalization go, you will be able to name your account to something completely different in the future if you decide to do so.