Translation:We already finished eating the dessert.
This one is quite pedantic. There are lots of ways to say it, but only one gets approval.
When someone is learning a language is should be as easy as possible for the learner without trying to figure out what is or is not accepable. I wonder how many people got discouraged and gave up becouse of your overly ridgid translation
This is a very common mistake in the course: If there is stress on the fact that the action has been completed, it kinda implies the usage of present perfect tense (happened in the (unspecified) past - overlaps into the present / we are talking about it + 'already' is almost always used with a perfect tense), but the system only accepts translations that use past simple tense. This is really disturbing, because thus DL requires us to write incorrect English translations.
'We HAVE already finished the dessert' should be accepted.
I think "We've already had dessert" has the same meaning, and could be accepted.
For regional and class reasons, a lot of people in Britain would expect "pudding" to be acceptable as an alternative to "dessert". Possibly also "sweet" (as a noun)
Mis-spelling "dessert" as "desert" should be accepted, but simply listed as a "typo" -- it is hard to remember which spelling corresponds to désert (dry area), desért (to abandon) or dessért (eaten after main portion of meal). In other words, a slight error like that should be forgiven in such a situation.
That's not how DL works: it accepts a misspelt word as a typo, if another word that it recognises doesn't come out of it. Remember that it's a computer programme, there's not a human teacher sitting on the other end. And while your point is understandable, the fact that it's difficult to remember which one is which isn't really an argument, all languages are full of pairs of similar words. Also, in your comment, you have just misspelt it three times: it's either 'desert' or 'dessert', English doesn't use accents above letters.