The Guardian has the details.

British actors will be barred from auditioning to play Prince William in a forthcoming film, because of new restrictions introduced after the country separates from the EU in January.

A
new casting notice asking for boys aged between nine and 12 years old,
who could conceivably play William at age 11, stipulates that only
European passport holders can apply. “NOT British-European,” warns the
note, “due to new Brexit rules from 1st January 2021.”