英语高手，来帮帮忙。谢谢了

that改为who
that are against you 是定语从句，先行词是many young men ，是人，关系代词应用who，that/which在代指物时用

语法上没看出错误，但这个句子没有必要有存在句型There be，这也是中学生容易犯的错误。
改一下，会不会好点？
Many young men are against you.

应为There are many young men that against you .
定语从句的“that”已经可以起到作用了。