Translation:They buy not only computers but also telephones.
How do we know that the sentence refers to plural computers and telephones?
Just because they is plural is no reason to assume the purchase of more than one device
"không những" means "not only" and is not a plural classifier here. plural is inferred from context.
but the nhung was only given for the computers, not for the telephones. The nhung/mot/a/the/plural "rules" seem extremely arbitrary.
In this case there's no better translation, if you have to choose between singular and plural, both are valid IF there really is a choice. Then you can also translate: "They buy not only a computer but also a phone." That's why it's irritating that plural is the only accepted condition here. But that's only valid IF THERE IS A CHOICE. :p
I think 'nhung' does not refer to the computers, it is part of a 'nhung ma' construction, in this case 'khong nhung ... ma con'. That still doesn't solve the problem of why plurals should be selected in this case, though. I may be wrong about the construction anyway, I'm pretty rubbish at Vietnamese!
Why is 'They buy not only a/the computer, but also a/the telephone' not correct here?
using 'mot' would indicate 'a' rather than 'the'. My question is really about why we should assume that computers and phones here are plural, when there is no plural indicator. Usually Duo always wants us to say the+singular unless we have the word 'mot' for one/a or nhung/cac for plurality. Here the nhung is part of a nhung...ma construction, not a plural indicator, so I want to know what indicates plurality here, and therefore why singular nouns with 'a' or 'the' are incorrect. Incidentally, in practice you don't need to say 'mot' to indicate that you mean 'a' instead of 'the', though Duo usually wants you to do that.
Computer and phone can be singular or plural - it depends on the situation/context. The Vietnamese sentence does not make the situation clear. But Duolingo accepts only one arbitrary answer. You are playing a guessing game with the application.