"Marjory is feeling warm."
Translation:Tha Marsaili a' faireachdainn blàth.
Because a' is short for ag which means 'at'. It is different from the a that is not an abbreviation, and means 'that' (relative pronoun). There is also a' that is short for an and means 'the'. There are specific rules for when you abbreviate, depending on what follows. It is not arbitrary or slang.
It's a computer program. Making a computer program behave like a human being on something as nuanced as grading exercises turns out to be really hard. That said, I've been trying to get the Scottish spelling of names, because so far, it's the orthography that's the real challenge of Gàidhlig; the grammar is peculiar, but easy.
There are three separate issues here.
Firstly, although Marjorie is a valid spelling in English, it is not one that Duolingo knows about, so far as I am aware.
Secondly, there has been a change of policy since they started writing the course. Originally, they expected you to translate names. So Marjory would have been translated as Marsaili. The policy has now changed and you are not expected to translate names. Logically, this means that Marjory should be translated as Marjory. But in fact any sentence with a non-Gaelic name in it can be assumed to be an old sentence from before the policy changed. So, for legacy reasons, you should always translate non-Gaelic names into Gaelic, although there should no longer be any necessity to translate when going in the other direction.
Thirdly, the apostrophe is before the space in a' faireachdainn as the bit that is missing is at the end of a', which is short of ag. The g is dropped before a vowel and replaced with the apostrophe.