The SQL plan cache overview includes statements from the XS engine and the statistics server/service.
These statements are processed differently in SAP HANA than 'normal' SQL commands that you can run via ODBC or JDBC connection in the indexserver.
Planviz and explain plan are only available for statements that run in the indexserver.
So, to 'fix' the issue, just pick a statement that has been run in the indexserver (check the service port number in the list) and run PlanViz on this statement then.