I came to understand that it was not easy to earn money, and knowledge could change my life.
这是一道改错题.答案是在 and 后和 knowledge 前加 that,请问为什么?可以省略这个 that 吗?

不能省略

I came to understand that it was not easy to earn money, and that knowledge could change my life.表示我明白了两件事：1，it was not easy to earn money 2，knowledge could change my life
这里的that提示这两个宾语从句是并列关系。

如果不加that，那就表示我只明白了一件事，即it was not easy to earn money ， 而后面knowledge could change my life则是作为一个完整的句子与I came to understand that it was not easy to earn money并列。

注意：两个宾语从句并列时，前一个that往往可以省略，而后一个则绝不可省略，否则会变成两个完整句并列，而非两个宾语从句并列。
如可将这句话省略第一个that变成：
I came to understand it was not easy to earn money, and that knowledge could change my life.=I came to understand that it was not easy to earn money, and that knowledge could change my life.