The 19-year-old high school student has been camping outside the British Embassy in Mexico City for nine days.
Chavez describes herself as a "royalty fan" and says she hopes for an invitation to the wedding of the year.
The British Embassy confirmed in a statement Friday that Chavez began a vigil last week. It said that "Buckingham Palace regrets that there are no more spots available."
Chavez says she believes that if the bride and groom hear of her effort, they will invite her to the April 29 wedding at London's Westminster Abbey.