致命错误：找不到类'plgContentWidgetkit_Content'

My website https://gutenberginstitute.com was developped using joomla and yootheme "venice". However, I have download and installed Widgetkit (free version so far) to test it on my site, and I can't make it work. It seems there are some plugins that are not installed, and if I put ... [widgetkit id=1] ... in some article, for example... was d