Error in train.default(x, y, weights = w, ...) :

[Qmoliang]

Error in train.default(x, y, weights = w, ...) :
At least one of the class levels is not a valid R variable name; This will cause errors when class probabilities are generated because the variables names will be converted to X0, X1 . Please use factor levels that can be used as valid R variable names (see ?make.names for help).

I think maybe I forgot something, I've installed anything of them but it still didn't work.