"They will have an hour" should be accepted :-(
I wrote "avranno un ora" which is exactly the answer they gave but was told I had a typo and "un ora" were each underlined.
Yes, un ora is mispelt, because an apostrophe is missing. The correct spelling is un'ora.
Without a context, this sentence can also translate as "They may have one hour" (conjectural use of the simple future).
They didn't give an apostrophe in the correction
You can report this as a mistake.