How to set thumbnail image and page description for sharepoint modern pages using pnpjs?

You can have a try below method to update thumbnail image and page description programmatically:

const page = await sp.web.loadClientsidePage("/sites/s01/SitePages/getcontrol.aspx");       
  page.description = "iam new desc";
  page.bannerImageUrl = "https://abc.sharepoint.com/sites/s01/testlib/Pictures/1.jpg";

  page.save();

More Reference:

• https://pnp.github.io/pnpjs/sp/clientside-pages/#load-using-iwebloadclientsidepage

BR