It's fine. Poser = to put/set/lay (sth) down. The English is a little forgiving in this context. Where are you going to put your suitcase? I'm going to put it down. Okay. The interesting thing about "contre" here is that we see it as "against" but here it is used more in the sense of near (or) close to. So I doubt that you are putting the suitcase down literally "against" the door (as if to block you or someone else from opening it), but putting it down by the door. Consider: ils étaient couchés l'un contre l'autre = they were lying close together. Source: Oxford Dictionary
"Luggage" is uncountable in English. But the similar words in French, such as "bagage" or "valise" work differently.
"Valise" seems to be the closest translation to "suitcase", and the plural works the same in French and English. "Bagage" is used in the plural when the meaning is the English "Baggage" or "Luggage": https://www.collinsdictionary.com/dictionary/french-english/bagage
For conjugating verbs in the imperative tense, verbs ending in 'ER' (including 'aller') follow these rules:
For the 'tu' form of imperative, as in saying "(you) go to school!" (a command), you would take the tu form of the verb conjugated in the present tense and take off the final 's'. I.e., Va à l'école! (Go to school!), and Mange tes nourritures! (Eat your food!).
For the 'nous' form of imperative, as in saying "let's go!", you just take the nous form of the verb conjugated in the present tense. I.e., Allons! (Let's go!), and Mangeons! (Let's eat!).
For the 'vous' form of imperative, it's the same as the 'vous' form of the verb conjugated in present tense. I.e., Allez! (Go! or Come on!).
The verbs 'être', 'avoir', 'vouloir', and 'savoir' have special rules for getting the imperative. For the 'tu', 'nous', and 'vous' imperatives for être, it's 'sois' and 'soyons', and 'soyez', respectively. For the corresponding forms for avoir, they are 'aie', 'ayons', and 'ayez'. For the corresponding forms for vouloir, they are 'veuille', 'voulions', and 'vouliez'. Finally, for the corresponding forms for savoir, they are 'sache', 'sachons', and 'sachez'.
All other verbs (i.e., IR and RE ending ones) use their 'tu', 'nous', and 'vous' forms in the present tense as their corresponding imperative forms.
Hope this helps!
No, in French you use "poser" to say you are putting/setting/placing something. From my knowledge it never actually means "keep" in french. (Or atleast i have never heard of it being used this way) You can say set your bag against the door, or set your bag by the door in English. Makes sense where I am from (Canada).
Actually, "ta valise" is singular and would refer to "your suitcase" or "your bag". Only the plural "tes valises" might be translated as "baggage". A single item is not "baggage". http://www.larousse.fr/dictionnaires/francais-anglais/valise/80038
The word "mettre" has so many meanings and does "put" in English. However, if you are talking about putting something down (on the floor/table, etc.), you would use "poser".
I assume you are referring to the "pick from a list of words" form of the exercise. We would really have to know what words you were given to understand if this is a bug in Duo (seems unlikely, given that I don't see any similar complaints here) or there is some other accepted way to form the sentence given the words provided.
Here is a good discussion of use and conjugation of the imperative: https://www.thoughtco.com/introduction-french-imperative-mood-1368858
You will also find the imperative conjugations for all verbs in this online French/English dictionary: https://www.collinsdictionary.com/dictionary/french-english/