I already arranged that error, didn’t remember to put Objeto as @Serializable, anyway getting more errors 
Grave: Uncaught exception thrown in Thread[LWJGL Renderer Thread,5,main]
java.lang.RuntimeException: Error serializing message
at com.jme3.network.base.MessageProtocol.messageToBuffer(MessageProtocol.java:81)
at com.jme3.network.base.DefaultClient.send(DefaultClient.java:237)
at com.jme3.network.base.DefaultClient.send(DefaultClient.java:207)
at mygame.ClientMain.simpleUpdate(ClientMain.java:81)
at com.jme3.app.SimpleApplication.update(SimpleApplication.java:242)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.runLoop(LwjglAbstractDisplay.java:151)
at com.jme3.system.lwjgl.LwjglDisplay.runLoop(LwjglDisplay.java:185)
at com.jme3.system.lwjgl.LwjglAbstractDisplay.run(LwjglAbstractDisplay.java:228)
at java.lang.Thread.run(Thread.java:744)
Caused by: com.jme3.network.serializing.SerializerException: Error writing object for field:private mygame.PlayerInventory mygame.messages.PlayerInventoryMessage.playerInventory
at com.jme3.network.serializing.serializers.FieldSerializer.writeObject(FieldSerializer.java:180)
at com.jme3.network.serializing.Serializer.writeClassAndObject(Serializer.java:389)
at com.jme3.network.base.MessageProtocol.messageToBuffer(MessageProtocol.java:73)
… 8 more
Caused by: com.jme3.network.serializing.SerializerException: Error writing object for field:private mygame.Objeto mygame.PlayerInventory.armaPrimaria
at com.jme3.network.serializing.serializers.FieldSerializer.writeObject(FieldSerializer.java:180)
at com.jme3.network.serializing.Serializer.writeClassAndObject(Serializer.java:389)
at com.jme3.network.serializing.serializers.FieldSerializer.writeObject(FieldSerializer.java:175)
… 10 more
Caused by: com.jme3.network.serializing.SerializerException: Error writing object for field:private com.jme3.scene.Spatial mygame.Objeto.spatial
at com.jme3.network.serializing.serializers.FieldSerializer.writeObject(FieldSerializer.java:180)
at com.jme3.network.serializing.Serializer.writeClassAndObject(Serializer.java:389)
at com.jme3.network.serializing.serializers.FieldSerializer.writeObject(FieldSerializer.java:175)
… 12 more
Caused by: java.lang.IllegalArgumentException: Class has not been registered:class com.jme3.scene.Geometry
at com.jme3.network.serializing.Serializer.getSerializerRegistration(Serializer.java:324)
at com.jme3.network.serializing.Serializer.getSerializerRegistration(Serializer.java:293)
at com.jme3.network.serializing.Serializer.writeClass(Serializer.java:368)
at com.jme3.network.serializing.Serializer.writeClassAndObject(Serializer.java:388)
at com.jme3.network.serializing.serializers.FieldSerializer.writeObject(FieldSerializer.java:175)
… 14 more
So i went on the forum and saw this old post:
I’ll try to refix this and post my results