I recall that Cody disallows the use of essentially all toolboxes. Symbolic variables are only active for you, IF you have the symbolic toolbox.
Why are they disallowed? Because that would not be fair to others. Not everyone has every toolbox. In fact many people might not have any specific toolbox. So Cody, to be fair to all users, only should allow solutions that use base MATLAB because otherwise you might find a solution that uses some function that nobody else will have. And that could not possibly be fair to others to compare to code they cannot access.
I can probably find a statement to this effect, were I to go looking for one in the help, but then I just found this Answer:
What Steven says, I would absolutely trust to be fact. Given that - Steve, should I buy or sell XXX stock? ;-)