The plugin only works with Java 8, and Java 9 currently. Java 10+ support is in progress. Recent Servoy Installers bundle the latest java version into it. To edit the java version, edit the servoy.ini file located in the developer folder of your Servoy installation. Add the below entry before the -vmargs in the file, adjusting the path as necessary. Note there may already be a -vm argument in the file, so just adjust the existing one.
More detailed notes on running Servoy/Eclipse in Java 9 can be found here: Configure Eclipse for Java 9
Servoy also has a relates article with information on changing the Java version: Java 9, 10, 11 support for the developer