Don’t know why @Janos deleted his answer, but it’s correct: your data frame
Train doesn’t have a column named
pre. When you pass a formula and a data frame to a model-fitting function, the names in the formula have to refer to columns in the data frame. Your
Train has columns called
quality. You need to change either your formula or your data frame so they’re consistent with each other.
And just to clarify:
Pre is an object containing a formula. That formula contains a reference to the variable
pre. It’s the latter that has to be consistent with the data frame.