Translation:A week is seven days.
This site is sometimes confusing and not understandable. One question asked "한 시간 ...", which I translated as "an hour"; Duolingo marked it wrong - they said it should be "one hour". OK, got it! So, this one I translated as "One week ...."; Duolingo said that is wrong - should be "a week ....". Go figure.
한 is the short form of the pure Korean (hangul) word 하나 for one.
일 is its Sino-korean (hanja, from Chinese) counterpart.
Hangul numbers are usually (not always) used for counting; hanja, for numbering.
Also compound words tend to be paired between words of the same origin i.e. hangul-hangul or hanja-hanja. For this reason, spacing is important sometimes in Korean.
Back to 한 and 일, both mean "one" and can both be used with 주, week (주간, hangul / 주일, hanja). Their meanings differ slightly in English.
한 주간, (with space between 한 and 주간) = a week
한 주간에 / 하루에 = weekly
일주일 (no space between 일 and 주일) = 1주 = one week
일주일에 = per week
일주일 = 한 주간 = 칠일 = 일곱 날 = (archaic) 이레