I never saw so betutiful lake when I was a young boy这句话哪里错了?

正确的应该是
I've never seen such a beautiful lake when I was young

你错在
1 时态有问题
2 时态和后面的从句不匹配
3 so不能这么用，能这么用的只有such
4 beautiful 拼错了
5 boy本身就有young的意思，不要重复表达

你没加过去时，而且，你的betutiful写错了，如你不加过去时，可以改为：
I never see so beautifui lake when i'm yong

I've never seen so beautiful a lake when I was a young boy.
OR
I never saw so beautiful a lake when I was a young boy.

there is a spelling mistake: betutiful-->beautiful

such a betutiful lake