You are writing a function that accepts 'temperature' as an input parameter. So, 'temperature' will exist inside your functions, assuming you pass in an argument to your function. 'temperature' is not declared on line 8, the existing values (line 1) is being overwritten.
That is all inside your function. Outside your function, working in the Command Window, 'temperature' will not exist unless you create it. Were you provided a set of test data? If so, you need to load that in and pass it to 'sortTemps', something like:
>> myData = ...
>> newData = sortTemps(myData);