If you have an enemy in a Zone of Truth who failed their save, they can still choose to not answer. If a player casts Command on them with the word being "answer" would that force the creature to answer the question posed? Similarly, what if you cast the command "lie" on a creature in a Zone of Truth?
The command spell is almost entirely left to the GM
The command spell states:
[...] Some typical commands and their effects follow. You might issue a command other than one described here. If you do so, the GM determines how the target behaves. If the target can't follow your command, the spell ends. [...]
Notably, "Answer" is not in the given list of commands and so anything that happens from this command is going to be up to the GM.
But even if it does force somebody to answer a question, and they are required to tell the truth, they can still choose the talk around the question or give an answer that is unclear. In fact, a GM could even argue "Dog" is an acceptable answer to a yes or no question because it is some kind of answer, just not a particularly helpful one.