In my experience, CodeIgniter error “Unable to load the requested class” is occurring because of naming conventions. First, you check the file is existing in the given path. Second the name of the file(Letter case).
Example: mcapi.php and MCAPI.php is not the same in most cases.
In Windows, letter case does matter but in Linux platforms it does. So if you are in Linux hosting, change the name of the file according to that.