Would it be correct to say "Can I have some sweets, please?"?
Yes. Perfectly correct!
Ah okay. I've reported the error, then.
And it's accepted now. (9 Apr)
How can you have your losin if you don't eat your cig?
I read that as "...eat your cig(arette)" ...
why us "May i have a sweet please?" incorrect?
It shouldn't be.
Wouldn't a single sweet be a losinen, though, rather than losin?
It can be, but a lot of people use losin for the singular and loshins for the plural. You'd probably get a funny look if you said Ga i losinen? in everyday Welsh.
What about "may I have sweets, please?"
Yes, but as shwmae notes earlier, losins would often be used for that as well.