I believe the intent here is to show us how to shout out "we can't hear you!", and that we wouldn't use poder (i.e., ¡No podemos oírte!) for this.
Otherwise, there isn't any good reason not to accept "we don't hear you" or "we didn't hear you," or even "we aren't hearing you."
Just the exclamation points, which clearly show this is an exclamatory statement. Of course, one could exclaim just about anything. The statement "we can't hear you" seems a natural for using "poder," but I've seen a few examples on Duo with the present tense that don't use "poder," even though we'd include it or a "helper" verb in English.
What David says makes sense. Often logic is good evidence.
For example, say a politician says one thing on the first day, and then the next day says he did not say that. This is good evidence the politician is a liar, or maybe is simply crazy.
A defender of that politician would say you don't have any evidence to draw that conclusion.
Could you say "we can't hear you" by saying "no podemos oirte?" And in a similar construction (with a different verb), could you say "no podemos encontrarlo" for "we can't find it?" Duolingo introduced this emphatic construction (alas, as usual with no explanation!) but didn't say whether this is the ONLY way to say that one can't hear or find something, or just the way to do it when you are mad and frustrated about it.
And could you use poder when you are frustrated about not hearing / finding ? Or do you HAVE to use the construction without poder to express frustration?
In general, whenever you see "can" or "may" in English, you can translate with the appropriately conjugated "poder." However, English often uses "can" as a helper verb without implying any sense of ability, permission or being able to do something. In those cases, you can omit "poder" in Spanish as not really necessary. Now, whether the cases you've described include ability is open to interpretation (i.e., the intent of the speaker). In other words, the speaker can decide to include or omit "poder" to communicate the desired idea. For example, one could be looking for something and say they're just not able to find it (they would include "poder") vs. they're actively looking but want to say they aren't finding it (they could omit "poder"). Annoyance and frustration could be present in both situations.
That would be "no, no te oímos." The "no" in the sentence we have been asked to translate is simply the negation of "oímos," it isn't the word "no." It's the equivalent of the "not" in "cannot" that you used in your translation. It isn't the equivalent of the "No" in your translation.