"I like dogs."
Translation:Is toil leam coin.
Absolutely correct. 'S is totally acceptable except in the most formal Gaelic. As to whether Duo accepts it, the notes say
Is toil vs. 'S toil
Is toil is quite commonly written as 's toil. The apostrophe represents a missing letter. Both are correct and they would generally be pronounced the same in a normal conversation.
So yes it should be accepted. As it hasn't you should report it. Because Duo has serious 'issues' with apostrophes that I don't think the writers knew how to deal with when the course was written that may have affected it but it should be put right if they can.