这句话总体上语法上没错,这个句子是that引导的表语从句.
Someone——主语
is——谓语
that I never want to lose and irreplaceable by anyone else——表语
主谓表结构,成分完整,时态语态正确,怎么错呢?
但是在从句中有结构不平衡的问题：
and这个连词前后的结构应该在形式和结构上是一致的,而原句中to lose和irreplaceable by anyone else显然不一致.
所以比较好的表达是：Someone is that I never want to lose and be replaced by anyone else.