Translation:The Åland Islands are islands in the Baltic Sea between Finland and Sweden.
"archipelago" should also be accepted as an alternative for "island group" or "islands".
That's skärgård in Swedish. Not all islands form archipelagos, after all.
Although Åland consists of many islands, they all make one place called Åland. In the same sense one could talk about "Swedens" or "Finlands" when referring to the countries with islands - which does not make sense.
How is this:'Åland is a group of islands, in the Baltic Sea, between Finland and Sweden' wrong? and this: 'Åland is islands in the Baltic Sea between Finland and Sweden.' is right - according to Duolingo? 'Åland is islands...' is not grammatically correct English.