"Marcus builds many cities."
Translation:Marcus multas urbes condit.
condō, condere, condidī, conditum is "to build" in the sense of "to found, to establish." Duo wants you to use condo with cities, which you build by a juridical act of synoecism (check out Wikipedia's article on Municipium.)
cōnstruō, cōnstruere, cōnstrūxī, cōnstrūctum is "build" in the civil engineering sense or actually laying bricks and raising up walls. Duo wants you to use construo with buildings, tables, and other such items.
No, multae would not work here.
Adjectives must match with the nouns they modify in case as well as number and gender. Since urbes are what is being built, it is the accusative (plural) case, so we must use the feminine accusative plural form of multus, multa, multum, which is multas.
If we urbes was doing the action (nominative) or being directly addressed (vocative) then multae would work.