Are you able to log in to the Stardog Cloud portal (https://cloud.stardog.com/)? If so, please go to the Manage Knowledge Graph Endpoints page, click the three dots to the right of your Stardog Free Knowledge Graph, and select the View Diagnostics Report option.
Do any of the tests on the left-hand side show an error? If so, we need to correct the problem. The most common issue we see is where the Service check fails, which means your web browser is unable to connect to the Stardog endpoint. We typically see this happen when a firewall (or other networking rule) blocks access from your location to the endpoint. In this case, please try accessing Stardog Cloud from a different network (e.g., using a VPN, or switching from your corporate work network to a home network).