Actually, an even better translation might be "My wife only ought white bread (and did not do anything else)".
If you want to emphasis that you are talking about buying but then want to restrict the buying to white bread only: "Mia edzina aĉetis nur blankan panon." (My wife bought only white bread (and nothing else)).
My translation "My wife has just bought some white bread" has been declined by Duolingo, NOT because it is wrong, but because Duo (being a wise old owl) said it should be "A white bread". That phrase makes no sense at all in standard English. Does anyone else suffer from these arbitrary decisions? I am frequently annoyed by such things, and sometimes wonder whether the folks at Duolingo really know their English sufficiently well to be teaching it.