If you followed 3. Unity plugin file import instruction, WebGL plugin is placed in /Assets/Plugins/WebGL. Unity automatically build with WebGL plugin. Other plugins shouldn't be included.
'XXX' could not be found. occurs if WebGL plugin is not correctly imported., 'XXX' already defined occurs if other plugins imported. Select plugin(JSClient.jslib, ProudNetClientPlugin-webgl.dll) - Inspector and check WebGL.
Uncheck Any Platform and other plugins not to include in WebGL build.