What can I do if my high school (or educational government authority in my country) will not sign and stamp the required school certificate form http://2wl.wum.edu.pl/en/node/831?

We accept a letter/document from your high school or educational government authority in your country that states that your diploma entitles you to undertake or continue university-level studies in the country in which your diploma was issued or in the system of education your school operates.