At first i wrote "we cannot find your telephone number" then I changed my mind as i remembered that for me DL usually wants a conjugation of "poder" in the spanish phrase before allowing the use of "can" in the translation. "we did not find your telephone number" was accepted.
Duo seems to be intentionally omitting poder in this kind of sentence/question, but still translating as "can't." See the "tips" for the School 4 lesson-- https://www.duolingo.com/skill/es/School-4/tips (keep scrolling down to encontrar and encontrarse). Just something else to learn, apparently!
This sentence can be either present tense or past tense.
The Spanish simple present tense and preterite tense is identical for the "we" conjugation of regular -AR and -IR verbs.
As far as adding "can", that is Duo's clumsy attempt to teach English speakers to infer implied words like "can, will, should" that are not actually stated.
I'm having trouble with that one.
I think too many people get caught up in the literal translations of things. Languages don't work that way.
English uses "can/can't" in a lot of sentences where it would sound awkward in Spanish. Yes, you could say it, but if it sounds awkward why would you? Juat because you say it in English doesn't mean you have to say it in Spanish.
I can say "I'm looking for your number but I can't find it" or "I'm looking for your number but I'm not finding it". These both mean the exact same thing, just different ways to say it English.
Now in Spanish, using poder here may not be a natural thing for a native speaker, maybe a native speaker can chime in and let us know. If that's the case then "Busco tu número, pero no lo encuentro" would be the best translation for both of my English sentences. I know Google Translate isn't the most reliable source, but when i put in "Busco tu número, pero no lo encuentro" the translation was "I'm looking for your number, but I can't find it".
My point is that we shouldn't try to push our English grammar into another language and that's the whole point of this example. It shows us that every time we see can/can't in a sentence does not mean the corresponding Spanish sentence will include poder and we as learners need to remember that.