Are you sure there was no other typo in your answer? Or, for example, a keyboard mixup?
Sometimes, learners forget to switch to the right keyboard until the lowercase letters show up, and the capital letter remains unchanged. If there id one letter that's not in the correct language, the sentencr is marked as wrong. If you are 100% sure that your answer was correct, please comment with a screenshot of your answer attached next time you come across this sentence and your answer gets marked wrong, so that we can report it.