We could delete this question, but perhaps I'll turn it into a bit of a lesson both on how to ask better questions, and more importantly, how to learn to diagnose problems that will inevitably arise when you use a system like TeX. Please don't take this answer as criticism but rather as hopefully helpful information for the future.
One of the most important things you can do to diagnose a problem is to create a Minimal Working Example which shows the problem that you asking about. We have plenty of information on how to do that on the Meta site. Once you've done that you should put that code (not a screenshot of your editor) in your question. This allows people to see what you are (or aren't) doing very quickly, and greatly improves you chances of getting a quick answer.
Now, the most minimal document that you could have created would have been the following:
But this document doesn't show the problem, so in effect it is too minimal. But had you created it, you would have seen immediately that you can in fact produce the character you were looking for.
So this leads quite quickly to the source of the problem in your actual document. Since there is a package loaded for a font, and you are having problems with the font, we can add that package to your document:
Now we see the source of the problem. Of course by doing this, you wouldn't have needed to ask the question. :-) (Or perhaps you could have asked a question about why the two documents behave differently.)
And the answer is:
As Robert notes in the comments, the GFSDidiot font is really designed for Greek and provides limited support for Latin characters. Because of this, it doesn't contain the Ć glyph, and it also seems that it isn't set up properly: