How do I convince professors that I have enough math for the PhD position?
Thanks for adding the text of the message you received.  The text makes it clear that this is a rejection, and it should be understood as final.  There is no point in following up to try to convince the professor that you should get the position; it will only annoy them.

The best thing you can do with this information is to improve your applications for other positions in the future.  It suggests that you need to improve your mathematics background if applying for positions like this one, and/or to review your application materials to make sure that they clearly demonstrate your preparation.