I'd not usually say 'prepare' for a single sandwich, I might if I were making a whole lot for a party or something - as in 'I've prepared the sandwiches for later'
A nicely old fashioned usage is 'cut' as well. My mother would say 'I've cut a round of sandwiches' where I'd say 'made a sandwich'. I don't think I'd ever 'cook' sandwiches though, even toasted ones.
This is a panino https://it.wikipedia.org/wiki/Panino
and this is a panino imbottito https://it.wikipedia.org/wiki/Panino_imbottito
and this is a sandwich https://it.wikipedia.org/wiki/Sandwich (Ummm, the last two look pretty much the same to me, note the pictures)
A panino is a bun or a roll, the ending tells us that it is a little bread)
A panino imbottito is the same as a panino, just more tastey an not so dry, because there is all kind of yummy stuff on it, just like on a sandwich.
Sandwich is the English word for a panino imbottito and the Italien (and English) word for those who prefer to refuse to speak Italian ;).
Either you are coming across some glitches in the system or you aren't reading the directions properly. It sounds like for the first one it wanted you to translate sandwich into Italian but you typed in the English word instead of the Italian. Likewise for the second one. It probably wanted you to translate the Italian into English but you typed the Italian out again. I've done it myself many times when I got in a hurry and didn't pay as much attention as I should have. Though every time I've done it I was informed that I typed it out in the wrong language.
If you are certain that you have been reading the directions correctly then report it and Duolingo will try to fix it for you.