How do I use this? When I enter an address in the search box it just comes up with "Error:". There is no map showing.

You need to allow location access, then it displays the map for your current location.

If you just want to view some places without giving location access, use this link: https://dev.explore.allmaps.org/

The hint text in the type in box says to give the URL of an old map, not the current name of a location