The following steps have worked for other users:
Remove any objects or elements on the slide that are not necessary, such as images or videos, and try exporting the slide again.
Check for any corrupted objects on the slide. Sometimes, a corrupted object can cause issues with saving or exporting the slide. To check for this, select all the objects on the slide and then press Ctrl + C to copy them. Create a new slide and then press Ctrl + V to paste the objects onto the new slide. Try exporting the new slide as a PDF.
Check if any objects on the slide have transparency or are overlapping. Overlapping objects or objects with transparency can cause issues with exporting. Try rearranging or adjusting the objects on the slide and then exporting again.
If the slide contains any animations or transitions, try removing them and then exporting the slide as a PDF.
Thanks,