Why we can't say " 拍照了一张照片“？

拍照 is the separate disyllabic verb (separable verb), here

https://www.cchatty.com/grammar/119879

A separate disyllabic verb cannot be directly followed by an object.

拍照了一张照片（NOK）

拍了一张照片（OK）

To put it simple, if you look at the word “拍照” , “拍” means “to take”， and“照” means “photos or pictures".

Therefore, you cannot say 拍照了一张照片, which means “took photos a photo”.

我拍了一张照片。I made a foto.(拍is verb。照片is noun。)

我拍照了。I made foto(s)

I made foto a foto. no. we do NOT say like that. 

somehow like you can't say:"my name is my name"

I hope that will help you. :)

可以说“拍一张照片”这没错，但是按照中文的说话习惯来说，“拍张照片”更合适。这只是说话习惯而已。