there is no - doubt that 为啥是表语从句

there is no doubt that 不是表语从句而是同位语从句。因为表语从句是放在系动词后面，而这一句that引导的从句放在名词doubt后说明名词的内容是同位语从句。

例如：there is no doubt that he can solve the problem by himself. 毫无疑问他自己能解决问题. 

名词 idea suggestion doubt problem等后面常常加同位语从句用来说明名词的内容

there is no doubt that 从句根本不是表语从句，而是同位语从句。中文意思是“亳无疑问/不可置疑/的确”

例如，毫无疑问，因为他工作出色，他被提拔为公司副总。

There is no doubt that he was promoted to vice president of the company because of his excellent work.