How do you say this in English (US)? question 1：
A: would you like more food?
B:no, I am full, I can't eat.
B:no, I am full, I can't stomach.
question 2:
I never can't eat/stomach seafood.
which is commonly used in US, why?

See a translation

question 1: would you like more food?
answer(我的回复）:
No thanks, I am full.
用 no thanks会比较有礼貌，犹如“不用了，谢谢”的意思。如果你要说你吃不下了，说“I can't eat anymore”。
I can't eat = 我不能吃。
anymore = 再多点。
I can't eat anymore= 我吃不下去了。
用"i can't eat"的话, 有可能会误解成你因为某种原因而不能吃食物。你也能说：
No thanks, I can't eat anymore.
这和：
No thanks, I am full
都有着相同意思。
不过：
No thanks, I am full, I can't eat.
可能会搞乱听者。
No thanks, I am full, I can't eat anymore.
可以用来强调你真的吃不下了。
不过第一和第二句的其中一句就行了。

我住在新加坡，所以从来没有听过I never stomach seafood。我搜了也找不到，所以不能确定。有可能是国外的用法吧。如果要说你从来不吃海鲜，可以说"I never eat seafood"（我从来不吃海鲜）或者稍微礼貌点"I don't eat seafood"（我不吃海鲜）。