Since the website doesn’t specifically address this, you (or your school administrator) could call National Merit and ask.
However, for SAT tests, College Board requires you to cancel scores due to illness by the Wednesday following the test administration. I’m not really sure if you can even cancel PSAT test scores, but I would think that waiting until the score was received has made that an impossibility. There are many students score very well on the SAT but do not make the NMSF cutoff on the PSAT.