minimum trace spacing error
In Pcbnew, click Tools -> DRC. This open the Design Rule Checking (DRC) tool. It checks trace spacing (clearance), as well as trace widths and via sizes. Minimum clearance between traces is set up in the Design Rules tool.

If you have errors, such as two traces being routed too close together, the tool will tell you where they are. Reroute the traces as needed and re-run the DRC. Keep doing this until you have zero errors. Never send a board to be manufactured without doing DRC.

You should also use the 'Check Unconnected' tool in the DRC to make sure you haven't missed any connections.