没有错...语法上没有错
He 主语 was absent from 谓语 school 宾语 because of the illness 原因状语从句.of介词后面跟名词...但是 the illness是指特定的疾病...如果你前面没有说明这个illness是什么 那么就会变得很奇怪...如果只有这么一句的话可以改成
He was absent from school yesterday because of his illness 或者
His absence from school yesterday was due to his illness 或者
He was absent from school yesterday because he was ill.