I Can't add node of a custom content type
After following clues mentioned in the above comments (by CodeNext), I was able to solve my problem. The issues and the solution I have compiled as below.

There were two problems.


For some needed reason, I displaced the "add content" menu from "Navigation" to "Redaction" (a new menu). This caused the new content type not appear in the "Add content" page. By displacing the new content type in the "add content" menu, I could see my new content type in the right place.
I had a underscore in the content type machine name. Because of this, i couldn't navigate to the node/add/newcontenttype page (I reached the node/add page). By removing it (the underscore), everything goes now the right way.