"I was making something to eat every evening" agrees with the online hints, is acceptable in English, and closer literally to the French presented. Shouldn't it be accepted?
Is this idiomatic?
Literally, it of course appears to be "I would make/prepare to eat every evening," but lacking in an object of said making/preparing.
DL- in some sentences, the 'soir' is translated as evening and sometimes as night. It is confusing!!
Well, sometimes soir means evening and sometimes it means night. Just do the best you can while you are learning the material. Some additional reading might help. If you use a search engine and type soir vs. nuit you will find a number of references in English.