I'm guessing that you might be trying to enter some answers in isolation rather than as parts of an overall mathematical sum (but if you're thinking of 'the radio times' then 'the' is redundant (I think 'radio times' is a better fit than the radio station I came up with - Radio X).
Similarly, are you trying to enter the sum of the clue number (28 across) 'squared', because that would be 28 letters if written out? If so, that's wrong - 'squared' applies to the overall sum (or parts of the sum - there are differences of opinion on how the maths works).'
The resulting sum should equal the puzzle's number 2525.