Make sure you have selected the correct target against which your project is compiled. It needs to be consistent with the Java version your Servoy install is built against. For this, do check Project > Properties > Java Compiler Node > JDK Compliance Panel.
Since the class which implements the IServerPlugin, ISmartClientPlugin or IClientPlugin is one file among many inside your jar, you should indicate which file is the plugin entry point.
The plugin jar can use Java Service Provider to expose Servoy Plugin classes. There should be a file inside the plugin jar at the path:
META-INF/services/com.servoy.j2db.plugins.IPlugin which which contains a line for each plugin that jar should expose (a class that implements class in the jar that implements IPlugin). The plugin should also have a default constructor (with no parameters). If file com.servoy.j2db.plugins.IPlugin is missing or contains invalid entries Servoy will automatically scan the jar for all classes that implement interface IPlugin. An example of file content (for whois plugin) is: