If it's marking you wrong, then you are entering something wrong, but since we can't see what you entered, it's hard to say exactly what. Do you have a screen shot so we can see what you typed? Or at least type it out here so if it's an error you are repeating we can see what you're missing?
I did go and check in the incubator to see if there were any reports. A few minutes after you typed this someone submitted a report on this one. The reports don't get linked to a specific user, but since it was around the same time, maybe it was you. That report had an error that the consonant tlh was being written (twice) with an uppercase i instead of a lowercase L. The font that Duolingo uses can be very frustrating. Two of the letters in this sentence ARE the uppercase i, but the letter in the middle of the tlh consonant is actually a lowercase L. Could that have been why you were being marked wrong?
I hope jdmcowan is right. I've seen a number of reports today with tIh instead of tlh, and I've been wishing we could reach out to that person. Thank you so much for coming and asking! There is never a tIh (although there can be a tIH, but that's different). Soon your fingers will learn to type tlh so easily that you won't be able to type health or triathlon correctly on the first try.