"The courier carries, but does not build a building."
Translation:Tabellarius portat, sed aedificium non construit.
But in good English, for both rhythym and clarity, you do try to use parallel constructions.
Good: "The courier carries, but does not build." (Neither verb specifies. The parallel gives clear meaning.)
Good: "The courier carries a letter, but does not build a building." (Both verbs specify. Again parallel with clear meaning.)
Bad: "The courier carries, but does not build a building." (Mixed constructions that could too easily be parsed with a bizarre meaning.)
We need a better explanation of the difference between condere and construere. They seem to overlap a little in meaning, but if I recall correctly, Duo only uses condere for cities and construere for roads and buildings. Is condere more about the planning and construere for the actual construction? I looked them up on Wiktionary, but it didn't really help.
"Aedificium" is used for the nominative case (subject), the accusative case (direct object) and the vocative case ("Salve, aedificium!"). "Aedificio" is used for both the dative case (indirect object) and the ablative case (after prepositions like "in", "ab", "cum", etc.). Here is a link to a table of the full declension of the word: (https://en.wiktionary.org/wiki/aedificium#Declension). And here is a page with a list of accusative and ablative prepositions (some use both cases): (https://en.wikiversity.org/wiki/Latin/Prepositions)