How polite is this? In English it sounds quite rude and demanding. Is this the prefered way of asking for something politely?
It's similar to English. It can either just be a simple statement, but if you went up to a person at a counter and said "Dw i eisiau siocled" you would be seen as a little rude or blunt. The polite form is "Ga i siocled, plîs/os gwelwch yn dda?". "May I have/ (some) chocolate, please?".
That is one of the alternative answers. Some people seem to be having problems with Duo not accepting valid alternative answers at present. The course teams cannot fix system problems, I am afraid, but there may be help on the Duo troubleshooting forums.