We hope to have more time to practise some knowledge (in which) we often make mistakes.
这里把that换成in which,相当于把你的关系副词换成了代词.
另外,其实that后面不一定要接一个句子.I love music that has a rock core.这种句子也没有错.
希望可以符合你老师的要求吧.
如果对你有帮助,望采纳.
另外不知道您这个more important突然加在后面是想表达什么,希望说的明确一点more important指的是some knowledge are more important这样的话就去掉and把more important放在前面就会通顺了。

We hope to have more time to practise some more important knowledge in which we often make mistakes.