How to reply this mail from potential PhD professor?

I doubt that any reply is expected. It seems to be a general response and may not have actually come from the professor, but from his/her office, instead.

I wouldn't expect much of any response until the deadline for application has passed. I suspect that viable candidates will then get further information about what else might be needed.

But if you haven't completed your application by submitting required materials, it would probably be good to do that soon.

Do not reply

That email can be categorized as a non-actionable notification email.

If you respond then you might paint yourself as a desperate, rude, or oblivious person.

• Desperate: You seek unnecessary affirmation
• Rude: You do not trust the "due course" which they mentioned
• Oblivious: You fail to understand that they are busy and would rather not receive inane follow-up emails

If you do not have a specific goal in your reply such as including crucial information which your application lacked then it's simply unneeded.