Your issue is that you have not told MATLAB what x is. Your function circular as it is written above has no idea what x, even if it is defined in another function or script as it does not exist in your functions workspace.
A simple solution is to pass x (if it exists elsewhere!) in as a argument to circular for example:
If for whatever reason you cannot pass function arguments, study the documentation for assigin.
As an aside, to diagnose errors such as this in the future, try using the debugger in MATLAB. For example you could place a breakpoint near the beginning of the function circular and you should see that x does not appear in the function workspace.