According to the Belgium state archives their civil registration records online only go up to 1920.
https://agatha.arch.be/On the family search catalogue the records for Nivelles do not cover such a recent date.
From familysearch wiki
Offices to Contact
For the more recent records (after 1915), the locality keeps one copy and the other copies are turned over to the State Archives (Algemeen Rijksarchief/Archives Générales du Royaume in Brussels, or Rijksarchief in de Provinciën/Archives de l'État in the provinces). Researchers can access them by contacting:
The National Archives
Algemeen Rijksarchief Ruisbroekstraat 2
Brussels 1000
BELGIUM
Telephone: 32 2 513 76 80
Fax: 32 2 513 76 81
Website