这个句子有没有语法错误？I can't believe it that I have to leave.

it 不去掉也可以，that引导同位语从句，与it 构成同位语成分

I can't believe it that I have to leave.→I can't believe that I have to leave.

believe加了句子就不要用宾语it了~~

把 it 删掉
此处that已经引导宾语从句I have to leave .作believe的宾语
it 多余了

按正常说法应该把it去了更顺一点, I can't believe that I have to leave.

it应去掉

去 it