IMHO "Morning and evening" shouldn't be accepted as a translation of "rano i wieczorem", because it means "poranek i wieczór".
1) Rano i wieczorem myję zęby -> I brush my teeth in the morning and in the evening
2) Rano (or 'poranek') i wieczór to pory dnia -> The morning and the evening are times of a day
The definite article is often dropped for 'Morning' and 'Evening' in English, and there are a few cases, as already mentioned, where the 'in' can be implied in English, so I think 'Morning and evening' should be accepted. If the aim is to teach us which case to use for these words on which occasions, it would be helpful to add some context and put them in a full sentence.
I think in most languages there is a lot of shorthand speech. In conversation an answer to a question about the time something is open or available or when something happens, the response morning and evening is just as appropriate and correct as in the morning and in the evening.