In PowerPoint Online, slide backgrounds extend to the edges of the slide. But when you print, a white margin appears around the edges of the slide. This is because most printers can’t do borderless printing.
Some printers have a "margin" option you can adjust that may make white margins smaller, but it still may not eliminate them entirely.
Select File > Print > Print Full Page Slides.
In the Print dialog box, look for a More settings link and select it.
Look for "Advanced" options or a "Margins" option that you can adjust, such as in the example below:
More about printers and margins
Most inkjet printers create irregular, nonprintable margins, with the margin that prints last) generally being wider than the margins on the other three sides.
When PowerPoint Online prints a slide, it sends an image of the slide to the printer at whatever size the printer and printer driver specify as the largest printable size. But PowerPoint Online can’t control where on the page the printer places the slide image.