Pourriez-vous, s'il vous plaît, épeler cela ?

English Translation

Could you spell that, please?

Why would be ça be incorrect here?

I believe cela and ça to be generally interchangeable, with the latter being more colloquial. In this polite sentence maybe cela is appropriate. Perhaps it also fits better with the rhythm of the sentence?

Ok so the question has an error since it marks ça as totally incorrect.