So it’s telling you that the expected scitype for a DecisionTreeClassifier is either Count or Continuous or Ordered Factored and that you’re feeding it something else (multi class) which is not ordered.
So what you should do is coerce the multi class so that they have an ordering. Or you could encode the multi class features first and then apply the decision tree.
What it’s doing here is that it’s effectively taking the multi class features as if they were ordered, using the ordering stored from the categorical array (lexicographic if I’m not mistaken); this may be fine but maybe not (which is why there’s a warning).
Note, possibly of interest: https://github.com/bensadeghi/DecisionTree.jl/issues/92