"awkward" is a little better than "embarrassing" maybe?  This doesn't sound like anything to be embarrassed about.

Today when I bought milk in the/a supermarket, I met a foreigner. He wanted to buy some steamed buns. (or "a steamed bun")But the clerk said “还没熟，不能吃”. He didn't know what means “还没熟” means/meant(past tense)，and wanted to know why he can't couldn't eat it.
He:Can you speak English？
I:it's raw ,you can't eat it .
He:why can't I eat it ？
I:。。。。。。。。I don't know how to explain
He:can you show me pinyin？ ("Can you show me the pinyin" or "Can you show me pinyin for this" sounds better.)
I：。。。。。。then
I show him the word "raw"  or "I showed him the word 'raw'"
He:Oh!raw.
Then I knew why he asked why he can't couldn't eat it again after I said "it's raw". (Without quotes"", this would be better "after I said it was raw.") I made a mistake in pronunciation. I said "row "not "raw"
It is because of this thing experience/exchange that I realized how terrible my spoken English is. I really want to improve my English. Could you help me? I can help with your Chinese.

I doubt your English is that bad.  He was probably just not thinking.  He should have been able to tell the word anyway.  "row" could be like "row a boat" or, same sound "roe" which means fish eggs.  But clearly the steamed bun was not fish eggs... and you can always try saying it a different way, like "uncooked" "not yet cooked'.  Actually, he probably studies Chinese a little and 书 is very commonly taught first. "shu".  And, like myself, he couldn't hear the tone for 熟 so he was thinking about this word, rather than what word you were trying to say which maybe he thought was 肉. Very short sentences are the easiest to misunderstand.  It's not even clear what language the words are in sometimes.