How could I delete DCIM sub folders in iphone
You simply can't delete them.

Your only option is to delete the subfolders/the images themselves. They need to be there, iOS does not allow you to delete them, therefore Windows shows an error message.

Why would you want to delete the folders anyway? If they contain the images, go ahead and delete those, but deleting a simple folder gives you basically no further storage at all.

This has been discussed on the Apple Forum as well.