Error in plot.rpart(model) : fit is not a tree, just a root

As an AI language model, I don't have the context of your code, so I'm going to provide a general answer to this error message.

This error message typically occurs when you try to plot a model that has only one node, or a root, and no branches. This often happens when you have a small dataset, or when the model is too simple to generate a tree structure.

To resolve this error, you can try to increase the complexity of the model or use a larger dataset. Alternatively, you can try to plot the model in a different way, such as using a bar chart or a heatmap.