Export the actual cutout file so transparency survives instead of capturing the checkerboard shown in the preview.
Use the export control
Once the result is ready, select Transparent in Download background and press Download PNG. This exports the image with an alpha channel, which tells compatible software which pixels are transparent. The checkerboard behind the preview is only a display aid. It is not part of the PNG generated by the download button.
A screenshot captures everything visible on the screen, including that checkerboard, the preview background and possibly the interface. Cropping the screenshot does not restore the original transparency. It also limits the image to the resolution at which the preview was displayed.
Check the saved artifact
Find the downloaded file and open it in a design app over a colored layer. The empty areas should reveal the layer underneath. A photo viewer that shows a white or black background is not by itself a failed export; different viewers display transparent pixels differently.
For a presentation, insert the PNG file rather than copying a screenshot from the webpage. Resize proportionally and check that no rectangular background appears over the slide.
Keep a reusable master
Save an unoutlined transparent copy if you plan several designs. Add white or custom backgrounds only to separate finished versions. This avoids having to remove a newly flattened background later. If saving fails in your browser, try the displayed result image's save option and then verify the downloaded format and transparency before continuing.