Standing by itself, the phrase "We received your invitation", is imperfect, and were I to try and translate it into French, I would have to use "Nous recevions..." Since we are asked here to translate from the French in the passé composé, we are given "avons reçu", the colloquial "lazy English" translation given omits the frankly needed helping verb, "have", as the strictly accurate "We have received your invitation" is what it should really be. By translating with uncommonly lazy English, Duolingo perpetuates and accelerates the decline of proper use of grammar, and I urge you to rethink your philosophy on it.
"We received" is the preterite in English. Though its connotation is a little different than "have recieved" (present perfect), it still denotes a completed past action.
The French imperfect often corresponds to English "used (infinitive)" or "was (present participle)" or "would (present). Occasionally the idea can also be expressed in English using the preterite or simple past form. But "received" cannot have an imperfect aspect because the preterite form denotes a once-and-done action. It's not an ongoing or repeated thing.
I urge you to rethink your philosophy on sounding stuck up and calling people lazy when you are completely, totally wrong. I don't know how you'd get the idea that "received" is imperfect, but most people are not both that smug and that wrong at the same time.
Actually I think Tom has a point, though perhaps he has been a little over forceful and I do not agree on his suggestion on the imperfect. In British English the form ‘we received’ would suggest further information is required. Eg - We received your invitation yesterday/ on Tuesday/ with great pleasure. As a stand alone sentence I would expect - We have received your invitation. However, Americans use the simple past in this way and this is an American site. But please, don’t mark us wrong when we use our correct British English.