Hi,
As the AI suggested, this issue can occur when multiple Virtual Desktops are active. If you are not intentionally using them, press Windows + Tab, then close the extra desktops so that only Desktop 1 remains, similar to the image below. Once the additional desktops are removed, restart your computer and check if you can enable the slideshow again.
If that does not fix the issue, the next fix requires editing the Registry as suggested from the thread below.
https://techcommunity.microsoft.com/discussions/edgeinsiderdiscussions/desktop-slideshow-doesnt-work-with-multiple-virtual-desktops/4168345If you're not familiar with editing the Registry, would you like me to guide you through the steps?