If a Scores app is on the same network as the Venue Server, a firewall is possibly blocking the connection to the Venue Server. Please refer to the related help article.
If the Scores app is not on the same network, and this is intended (ie. several courts using a different network for example), it is possible to ignore a Venue Server by going into the menu of the Scores app (top-right) and click the Ignore Venue Server button.
Note: While trying to connect to an active Venue Server, a Scores app will still be connected to the fiba3x3 servers, no data will be lost during that time.