It's mostly correct, although the second sentence in part B is a bit awkward. A better way to say it might be: A: What kind of food is this? B: I'm not sure. I don't think it can be Thai or Indonesian. or A: What kind of food is this? B: I'm not sure, but I am certain that it is not Thai or Indonesian!

I think your sentence sounds VERY natural in informal, spoken English (I am British). The grammar might not be perfect, but this is still how people speak! You just need to add "don't" (because it's a negative statement: "...can't be..."). The last bit ("I don't think"/"I think") would usually be said as a rhetorical question. A What kind of food is this? B I'm not sure. It can't be Thai or Indonesian, I don't think?!

Both sentences are correct, but the last one sounds a bit better the other way around: "I'm not sure. I think it can't be Thai or Indonesian. " Hope it helps!