another master's or PhD?
Although it is odd for universities to ask you this, assuming that they did 
you have several options


Attend credit courses pertaining to Machine Learning or Artificial Intelligence and re apply
Get some industrial experience in those fields and re apply
Select a program that does not require Machine Learning background.
Talk to dept advisers telling them that you will attend these courses 
when you are admitted, probably get a conditional acceptance or 
you do not want to do PhD in machine learning fields but something else
(assuming that is the case) and approach the faculty member to back you up.