Yes, you can use ann and an sin to translate there but this is nothing to do with the two being interchangeable. It is just that there has two meanings in English. Its primary meaning is to say where something is, but it can also be used to indicate that something exists:
|Tha cat an seo||Here's a cat (pointing)||There's a cat here|
|Tha cat an sin||There's a cat (pointing)||There's a cat there|
|Tha cat an siud||There's a cat (pointing far away)||There's a cat there in the distance|
|Tha cat anns an sgeul||-||There's a cat in the story|
|Tha cat air a' mhata||-||There's a cat on the mat|
|Tha cat ann||-||There's a cat (in existence)|
But you will notice the confusion because the third column shows that in English you sometimes have the option to use an extra existential there at the beginning - and sometimes it is even necessary. We do not do this in Gaelic. If there is anything in the sentence to show where the cat is then we do not need the ann to show the cat exists. It is only if there is no clue to location that we use Tha ... ann = 'there's' to show that something exists, like the French il y a or the German es gibt (neither of which can be confused with a location, although they can be used with a location). (Exceptionally, if you really wanted to emphasise your presence, you could say Bha mi ann an sin 'I was actually present there'.)
So I hope I have made it clear that the problem is with the English and that you only use ann by itself to indicate existence, when not accompanied by a location.
In this particular sentence they would both be valid translations, but answering different questions. Ann would answer the question 'Were you there yesterday afternoon?', putting the emphasis on your existence at that time, effectively saying 'Yes, I was present.' But an sin would emphasise where you were, so perhaps you are pointing to where you were, perhaps in answer to the question 'Where were you yesterday afternoon?'
Note that there is no hyphen in an sin etc. (places) but there is in an-dè etc. (times). This has only recently been standardized so you may still variation, even on Duolingo. It is discussed here.
Note that there are other uses of ann (not covered yet) that could potentially be followed by an sin etc. D
Well hopefully my answer will help a bit. The mods don't generally read these pages unless either
- You report an error using the 'report' button, or
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It would be very helpful if there were some way to flag a comment for the mods' attention.
You have just answered your own question. If it changes the emphasis it is not an accurate translation, so you should not use it unless the rules of English do not permit a more accurate translation.
But there is a practical reason too. The volunteers who write this course spend hours putting in all the reasonable answers they think people might give. Now if they had to allow for an unnecessary swapping round of two parts it would literally double the number of answers that were required. So there is, in practice, a rule that arbitrary changes of word order, that is ones not require by the grammar of the target language, should not be used.