Error is exactly what it says--you wrote:
X = categorical({'March','April','May','June','July','August','November'});
M = reordercats(X,{'March','April','May','June','July','August','November'});
so M is a categorical variable which polyfit is not equipped to handle.
Use
>> M=month(datetime(string(X),'inputformat','MMM'))
M =
3 4 5 6 7 8 11
>>
or some other way to generate the months as numeric.
I THINK some of the Statistics and/or Curve Fitting toolbox routines may know how to handle categorical variables but I don't recall for absolute sure otomh.